Common questions about Worcester Bosch codes · Search all brands by code or symptom
What is safe for you to do
Every code here follows the same safety rules. You can:
- press the reset button once, as your user guide shows;
- for low-pressure codes, top up the pressure with the filling loop, as your user guide shows;
- for ignition and flame-loss codes, check the gas is on at the meter and a prepayment meter has credit;
- thaw a frozen plastic condensate pipe outside with warm (not boiling) water;
- check the thermostat, timer and power to the boiler.
Never open the boiler casing. Only a Gas Safe registered engineer may work inside a gas boiler. Codes marked Engineer after one reset have no homeowner fix: reset once, then leave them to an engineer. Codes marked Safe checks first list the one check that relates to that code.
Greenstar i ErP (Greenstar 25i / 30i combi)
Greenstar i boilers show a two-character fault code (for example EA) and a three-digit cause code (for example 227). Quote both, e.g. 'EA 227'. The same layout is used across many Greenstar models, but always check your own manual.
Source: Greenstar i ErP Installation, Commissioning and Servicing Instructions, doc. 6 720 806 944 (2016/07), pp. 65–68, section 8.1.2 Error codes (Table 48). Open the manual (PDF) (Manufacturer copy)
Ignition / flame
FA 306 Worcester Bosch Greenstar i
Engineer after one resetIgnition / flameWhat it means
A flame (ionisation) signal was detected after the gas valve closed.
Likely causes
Faulty ionisation probe or a gas valve that is not shutting off properly.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does FA 306 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, FA 306 means a flame (ionisation) signal was detected after the gas valve closed. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
EA 227 Worcester Bosch Greenstar i
Safe checks firstIgnition / flameWhat it means
No flame was detected after ignition, so the boiler locked out.
Likely causes
No gas or low gas pressure, a frozen or blocked condensate pipe, a restricted flue, an incorrect gas/air ratio, a worn ignition unit or electrodes, a gas valve fault, or a dirty appliance.
What you can safely do
Check another gas appliance such as the hob works, the gas meter valve is on and, on a prepayment meter, there is credit. In freezing weather, check the plastic condensate pipe outside: if it is frozen, pour warm (not boiling) water along it. Then press reset once. If the code comes back, stop resetting.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the gas supply is fine and the code returns after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does EA 227 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, EA 227 means no flame was detected after ignition, so the boiler locked out. Check another gas appliance works, the meter valve is on and any prepay meter has credit, then reset once. If it returns, book a Gas Safe registered engineer.
F7 228 Worcester Bosch Greenstar i
Engineer after one resetIgnition / flameWhat it means
A flame signal was detected before the burner started.
Likely causes
A damaged or poorly connected ionisation probe, or condensate not draining properly.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does F7 228 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F7 228 means a flame signal was detected before the burner started. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
EA 229 Worcester Bosch Greenstar i
Engineer after one resetIgnition / flameWhat it means
The flame signal was lost while the boiler was running.
Likely causes
Incorrect gas/air ratio, low dynamic gas pressure, flame probe wiring or probe wear, or a blocked condensate pipe.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does EA 229 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, EA 229 means the flame signal was lost while the boiler was running. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 238 Worcester Bosch Greenstar i
Engineer after one resetIgnition / flameWhat it means
Gas valve or control box error.
Likely causes
No supply to the gas valve coils, a failed gas valve, or a control box fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does F0 238 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 238 is listed in the manufacturer's table as "Gas valve or control box error". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Water pressure / circulation
A1 281 Worcester Bosch Greenstar i
Engineer after one resetWater pressure / circulationWhat it means
The pump appears to be stuck or running dry.
Likely causes
Low system pressure, air in the system, poor circulation or a pump or pump wiring fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if you have to top up more than once every few months, the gauge goes above about 2.5 bar when hot, water drips from the outside overflow pipe, or the code returns after topping up.
What does A1 281 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, A1 281 means the pump appears to be stuck or running dry. Reset once; if the code comes back, book a Gas Safe registered engineer and quote the full code. Do not open the casing.
Temperature / overheat
E9 276 Worcester Bosch Greenstar i
Engineer after one resetTemperature / overheatWhat it means
The primary flow temperature went above 95 °C.
Likely causes
Low system pressure, air in the system, poor circulation or a pump fault.
What you can safely do
Let the boiler cool for 20 to 30 minutes, then press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Overheat shut-downs are safety devices doing their job, so this is engineer-only.
What does E9 276 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, E9 276 means the primary flow temperature went above 95 °C. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
E9 224 Worcester Bosch Greenstar i
Engineer after one resetTemperature / overheatWhat it means
The flue thermostat or the high-limit (overheat) thermostat has opened.
Likely causes
Poor circulation, a faulty thermostat or wiring, flue gases leaking past the burner seal, debris in the heat exchanger or an incorrect air/gas ratio.
What you can safely do
Let the boiler cool for 20 to 30 minutes, then press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Overheat shut-downs are safety devices doing their job, so this is engineer-only.
What does E9 224 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, E9 224 means the flue thermostat or the high-limit (overheat) thermostat has opened. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
D4 341 Worcester Bosch Greenstar i
Engineer after one resetTemperature / overheatWhat it means
The primary flow temperature is rising too quickly.
Likely causes
Low pressure, air in the system, poor circulation, or a pump or sensor fault.
What you can safely do
Let the boiler cool for 20 to 30 minutes, then press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Overheat shut-downs are safety devices doing their job, so this is engineer-only.
What does D4 341 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, D4 341 means the primary flow temperature is rising too quickly. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Flue / fan / air
C7 214 Worcester Bosch Greenstar i
Engineer after one resetFlue / fan / airWhat it means
The fan did not run during start-up.
Likely causes
Fan wiring or connector, no supply to the fan, a failed fan, or the control box.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does C7 214 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C7 214 means the fan did not run during start-up. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
C6 215 Worcester Bosch Greenstar i
Engineer after one resetFlue / fan / airWhat it means
The fan is running too fast.
Likely causes
Fan wiring, a failing fan, or the control box.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does C6 215 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C6 215 means the fan is running too fast. Likely causes include fan wiring, a failing fan, or the control box. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
C6 216 Worcester Bosch Greenstar i
Engineer after one resetFlue / fan / airWhat it means
The fan is running too slowly.
Likely causes
Fan wiring, a failing fan, or the control box.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does C6 216 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C6 216 means the fan is running too slowly. Likely causes include fan wiring, a failing fan, or the control box. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
C7 217 Worcester Bosch Greenstar i
Engineer after one resetFlue / fan / airWhat it means
The fan stopped while the boiler was running.
Likely causes
Fan wiring or supply, a failed fan, or the control box.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does C7 217 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C7 217 means the fan stopped while the boiler was running. Likely causes include fan wiring or supply, a failed fan, or the control box. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
C1 264 Worcester Bosch Greenstar i
Engineer after one resetFlue / fan / airWhat it means
The fan stopped during operation.
Likely causes
Fan wiring, fan supply or a failed fan.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does C1 264 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C1 264 means the fan stopped during operation. Likely causes include fan wiring, fan supply or a failed fan. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Sensors / thermistors
E2 222 Worcester Bosch Greenstar i
Engineer after one resetSensors / thermistorsWhat it means
The flow temperature sensor reads as a short circuit.
Likely causes
Failed sensor or damaged harness.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does E2 222 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, E2 222 means the flow temperature sensor reads as a short circuit. Likely causes include failed sensor or damaged harness. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
E2 223 Worcester Bosch Greenstar i
Engineer after one resetSensors / thermistorsWhat it means
The flow temperature sensor is disconnected.
Likely causes
Unplugged or failed sensor or a broken wire.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does E2 223 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, E2 223 means the flow temperature sensor is disconnected. Likely causes include unplugged or failed sensor or a broken wire. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
E2 350 Worcester Bosch Greenstar i
Engineer after one resetSensors / thermistorsWhat it means
Flow temperature sensor short circuit (second sensor code).
Likely causes
Failed sensor or damaged harness.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does E2 350 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, E2 350 is listed in the manufacturer's table as "Flow temperature sensor short circuit (second sensor code)". Likely causes include failed sensor or damaged harness. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
E2 351 Worcester Bosch Greenstar i
Engineer after one resetSensors / thermistorsWhat it means
Flow temperature sensor disconnected (second sensor code).
Likely causes
Unplugged or failed sensor or a broken wire.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does E2 351 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, E2 351 is listed in the manufacturer's table as "Flow temperature sensor disconnected (second sensor code)". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
CC 800 Worcester Bosch Greenstar i
Engineer after one resetSensors / thermistorsWhat it means
Outdoor weather sensor fault (only shown when an outdoor sensor accessory is connected).
Likely causes
Sensor wiring, connector or a failed sensor.
What you can safely do
Nothing urgent: the boiler normally keeps heating without the outdoor sensor, using a fixed setting. Press reset once to see if it clears.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does CC 800 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, CC 800 is listed in the manufacturer's table as "Outdoor weather sensor fault (only shown when an outdoor sensor accessory is connected)". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
A7 808 Worcester Bosch Greenstar i
Engineer after one resetSensors / thermistorsWhat it means
Hot water temperature sensor fault.
Likely causes
Sensor wiring, connector or a failed sensor.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does A7 808 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, A7 808 is listed in the manufacturer's table as "Hot water temperature sensor fault". Likely causes include sensor wiring, connector or a failed sensor. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
PCB / electrical
Fd 231 Worcester Bosch Greenstar i
Safe checks firstPCB / electricalWhat it means
The power was interrupted while the boiler was already locked out. The previous lockout code is the real fault.
Likely causes
A power cut or someone switching the boiler off during a lockout. Check the code shown before Fd.
What you can safely do
Look for the code shown before Fd and use that code's advice. If the gas is on and pressure is fine, press reset once.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the boiler locks out again, quoting the original code.
What does Fd 231 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, Fd 231 means the power was interrupted while the boiler was already locked out. The previous lockout code is the real fault. Reset once; if the code comes back, book a Gas Safe registered engineer and quote the full code. Do not open the casing.
EA 261 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The boiler's code plug (HCM) may be faulty.
Likely causes
An HCM not fully inserted or the wrong HCM, or a blocked condensate pipe.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does EA 261 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, EA 261 is listed in the manufacturer's table as "The boiler's code plug (HCM) may be faulty". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
9U 233 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or the code plug (HCM) is faulty or loose.
Likely causes
Loose or failed HCM, or a control box fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 9U 233 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, 9U 233 means the control box or the code plug (HCM) is faulty or loose. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
C4 237 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose or failed HCM, or a control box fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does C4 237 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C4 237 means the control box or code plug (HCM) is faulty. Likely causes include loose or failed HCM, or a control box fault. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 239 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose or failed HCM, or a control box fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 239 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 239 means the control box or code plug (HCM) is faulty. Likely causes include loose or failed HCM, or a control box fault. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 242 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty (the manual groups cause codes 242 to 256 together).
Likely causes
Loose HCM or control box connections; the manual says to contact Worcester's technical helpline.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 242 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 242 means the control box or code plug (HCM) is faulty (the manual groups cause codes 242 to 256 together). There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
b7 257 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose HCM or control box connections.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does b7 257 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, b7 257 means the control box or code plug (HCM) is faulty. Likely causes include loose HCM or control box connections. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 258 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty. The manual also lists this cause with display code EH.
Likely causes
Loose HCM or control box connections.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 258 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 258 means the control box or code plug (HCM) is faulty. Likely causes include loose HCM or control box connections. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F1 259 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose HCM or control box connections.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F1 259 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F1 259 means the control box or code plug (HCM) is faulty. Likely causes include loose HCM or control box connections. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 262 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose HCM or control box connections.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 262 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 262 means the control box or code plug (HCM) is faulty. Likely causes include loose HCM or control box connections. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F1 263 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose HCM or control box connections.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F1 263 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F1 263 means the control box or code plug (HCM) is faulty. Likely causes include loose HCM or control box connections. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 272 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box or code plug (HCM) is faulty.
Likely causes
Loose or failed HCM, or a control box fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 272 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 272 means the control box or code plug (HCM) is faulty. Likely causes include loose or failed HCM, or a control box fault. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 280 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The control box is faulty.
Likely causes
Loose connections or a failed control box.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 280 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 280 means the control box is faulty. Likely causes include loose connections or a failed control box. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
F0 290 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
Control box fault (analogue/digital reference error).
Likely causes
Loose burner control connections or a failed burner control.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does F0 290 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, F0 290 is listed in the manufacturer's table as "Control box fault (analogue/digital reference error)". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
A8 361 Worcester Bosch Greenstar i
Engineer after one resetPCB / electricalWhat it means
The fitted code plug (HCM) does not match the control box or display.
Likely causes
Wrong HCM fitted after a repair.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does A8 361 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, A8 361 means the fitted code plug (HCM) does not match the control box or display. Likely causes include wrong HCM fitted after a repair. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Other / controls
C4 273 Worcester Bosch Greenstar i
Safe checks firstOther / controlsWhat it means
The boiler has run continuously for 24 hours and is pausing for a short self-check.
Likely causes
A heat demand that has been on non-stop for 24 hours. The boiler stops for at least 2 minutes then carries on.
What you can safely do
Nothing to do. The boiler pauses for a few minutes after 24 hours of non-stop running and then carries on.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer only if the boiler does not restart by itself.
What does C4 273 mean on a Worcester Bosch Greenstar i ErP boiler?
On a Worcester Bosch Greenstar i ErP, C4 273 means the boiler has run continuously for 24 hours and is pausing for a short self-check. This is a routine pause rather than a fault, and the boiler should restart by itself after a short time. If it shows for long or with other codes, book a Gas Safe registered engineer.
Greenstar 4000 (GR4700iW C combi)
The Greenstar 4000 shows a number (for example 227) with a short text description. A 'V' fault locks the boiler until it is reset; a 'B' fault clears itself when the cause goes away.
Source: Greenstar 4000 Installation and Maintenance Instructions (GR4700iW C), doc. 6 720 891 161 (2021/10) UK/IE, pp. 71–75, section 10.1.2 Operating/Fault code table. Open the manual (PDF) (Manufacturer copy)
Ignition / flame
227 Worcester Bosch Greenstar 4000
Safe checks firstIgnition / flameWhat it means
The boiler tried to light but did not detect a flame after ignition, so it locked out.
Likely causes
No gas or low gas pressure, a frozen or blocked condensate pipe, a blocked flue or air intake, or a fault with the spark electrode, flame sensor, gas valve or burner.
What you can safely do
Check another gas appliance such as the hob works, the gas meter valve is on and, on a prepayment meter, there is credit. In freezing weather, check the plastic condensate pipe outside: if it is frozen, pour warm (not boiling) water along it. Then press reset once. If the code comes back, stop resetting.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the gas supply is fine and the code returns after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 227 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 227 means the boiler tried to light but did not detect a flame after ignition, so it locked out. Check another gas appliance works, the meter valve is on and any prepay meter has credit, then reset once. If it returns, book a Gas Safe registered engineer.
228 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
The boiler is seeing a flame signal when there should not be a flame.
Likely causes
A faulty or damp flame sensor, an earthing problem, water collecting in the condensate trap area, a faulty gas valve or a control board fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 228 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 228 means the boiler is seeing a flame signal when there should not be a flame. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
234 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
The boiler has detected an electrical fault with the gas valve.
Likely causes
Gas supply problem at the valve, loose gas valve connections, a damaged harness or a failed gas valve.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 234 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 234 means the boiler has detected an electrical fault with the gas valve. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
269 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
A fault has been detected in the flame monitoring circuit.
Likely causes
Wiring or board connection problem, or a faulty control board.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 269 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 269 means a fault has been detected in the flame monitoring circuit. Likely causes include wiring or board connection problem, or a faulty control board. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
306 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
A flame signal is still detected after the gas supply has been shut off.
Likely causes
Gas valve not closing fully, faulty electrodes or leads, or a control board fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 306 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 306 means a flame signal is still detected after the gas supply has been shut off. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
363 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
System fault in the boiler electronics linked to flame (ionisation) measurement.
Likely causes
Damaged ionisation lead, worn electrodes, or a control unit fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 363 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 363 is listed in the manufacturer's table as "System fault in the boiler electronics linked to flame (ionisation) measurement". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2916 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
The gas valve was detected as open during the post-purge period.
Likely causes
A gas valve that is not sealing or a control board fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 2916 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2916 means the gas valve was detected as open during the post-purge period. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2920 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
Fault in the flame monitoring system.
Likely causes
Worn electrodes or leads, a poor earth connection, or the control unit.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 2920 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2920 is listed in the manufacturer's table as "Fault in the flame monitoring system". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2923 Worcester Bosch Greenstar 4000
Engineer after one resetIgnition / flameWhat it means
Unexpected feedback from the modulating gas valve.
Likely causes
Gas valve wiring or coil fault, or a control unit fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Ignition and flame faults involve gas, so this is engineer-only.
What does 2923 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2923 is listed in the manufacturer's table as "Unexpected feedback from the modulating gas valve". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Water pressure / circulation
281 Worcester Bosch Greenstar 4000
Engineer after one resetWater pressure / circulationWhat it means
The pump appears to be stuck or running with air instead of water.
Likely causes
Low system pressure, trapped air, poor circulation in the heating system, or a pump or pump wiring fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if you have to top up more than once every few months, the gauge goes above about 2.5 bar when hot, water drips from the outside overflow pipe, or the code returns after topping up.
What does 281 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 281 means the pump appears to be stuck or running with air instead of water. Reset once; if the code comes back, book a Gas Safe registered engineer and quote the full code. Do not open the casing.
1017 Worcester Bosch Greenstar 4000
Safe checks firstWater pressure / circulationWhat it means
Maintenance warning: system water pressure is too low.
Likely causes
Water lost from the heating system (leak, pressure relief valve, recently bled radiators) or a faulty pressure sensor.
What you can safely do
Look at the pressure gauge while the boiler is cold. If it is below about 1 bar, top it up with the filling loop exactly as your user guide shows, to about 1–1.5 bar or the figure in your guide, then close the loop fully. Check the radiator valves are open, then press reset once.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if you have to top up more than once every few months, the gauge goes above about 2.5 bar when hot, water drips from the outside overflow pipe, or the code returns after topping up.
What does 1017 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1017 is a maintenance warning: system water pressure is too low. With the boiler cold, check the pressure gauge; if it is below the level in your user guide, top it up as the guide shows, then reset once. If it returns, book a Gas Safe registered engineer.
Temperature / overheat
224 Worcester Bosch Greenstar 4000
Engineer after one resetTemperature / overheatWhat it means
A safety overheat device on the flue or heat exchanger has tripped. Short-lived it blocks the boiler; if it persists it becomes a lockout.
Likely causes
Low system pressure, air in the system, a seized pump, a blockage restricting water flow, or a faulty limit thermostat or its wiring.
What you can safely do
Let the boiler cool for 20 to 30 minutes, then press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code comes back after one reset. Overheat shut-downs are safety devices doing their job, so this is engineer-only.
What does 224 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 224 means a safety overheat device on the flue or heat exchanger has tripped. Short-lived it blocks the boiler; if it persists it becomes a lockout. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Flue / fan / air
214 Worcester Bosch Greenstar 4000
Engineer after one resetFlue / fan / airWhat it means
The fan stopped while the boiler was in its ignition safety period, so the boiler locked out.
Likely causes
A fan wiring or supply problem, the fan itself, or the control board.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does 214 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 214 means the fan stopped while the boiler was in its ignition safety period, so the boiler locked out. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2910 Worcester Bosch Greenstar 4000
Engineer after one resetFlue / fan / airWhat it means
The boiler has detected a problem in the flue system.
Likely causes
An obstruction in the flue, a failed flue integrity check, or water sitting in the sump because the condensate trap (siphon) is not draining.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does 2910 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2910 means the boiler has detected a problem in the flue system. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2915 Worcester Bosch Greenstar 4000
Engineer after one resetFlue / fan / airWhat it means
The fan keeps running after the post-purge period has ended.
Likely causes
Fan supply, wiring or speed control fault.
What you can safely do
There is no homeowner check that fixes this code. Write down the full code, press reset once, and if it comes back leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the terminal looks clear and the code returns. Flue and fan faults can affect how combustion gases leave your home, so this is engineer-only.
What does 2915 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2915 means the fan keeps running after the post-purge period has ended. Likely causes include fan supply, wiring or speed control fault. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Sensors / thermistors
1021 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
Fault with the hot water temperature sensor; hot water is affected.
Likely causes
Sensor disconnected, fallen off its pipe, failed or a damaged harness. The manual says it only clears with a power off/on after the sensor is fixed.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1021 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1021 is listed in the manufacturer's table as "Fault with the hot water temperature sensor; hot water is affected". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
1037 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
Outdoor (weather) sensor fault; the boiler runs in a back-up heating mode.
Likely causes
Weather compensation switched on without a sensor fitted, a badly positioned sensor, or loose sensor wiring.
What you can safely do
Nothing urgent: the boiler normally keeps heating without the outdoor sensor, using a fixed setting. Press reset once to see if it clears.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1037 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1037 is listed in the manufacturer's table as "Outdoor (weather) sensor fault; the boiler runs in a back-up heating mode". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
1065 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
The system pressure sensor is faulty or not connected.
Likely causes
Unplugged sensor, a broken or shorted sensor lead, or a failed pressure sensor.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1065 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1065 means the system pressure sensor is faulty or not connected. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
1073 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
The flow temperature sensor reads as a short circuit.
Likely causes
Failed flow sensor or damaged wiring.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1073 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1073 means the flow temperature sensor reads as a short circuit. Likely causes include failed flow sensor or damaged wiring. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
1074 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
No signal from the flow temperature sensor (open circuit).
Likely causes
Unplugged or failed flow sensor or a broken wire.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1074 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1074 is listed in the manufacturer's table as "No signal from the flow temperature sensor (open circuit)". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
1075 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
The heat exchanger temperature sensor reads as a short circuit.
Likely causes
Failed sensor or damaged wiring.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1075 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1075 means the heat exchanger temperature sensor reads as a short circuit. Likely causes include failed sensor or damaged wiring. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
1076 Worcester Bosch Greenstar 4000
Engineer after one resetSensors / thermistorsWhat it means
The heat exchanger temperature sensor is disconnected.
Likely causes
Unplugged or failed sensor or a broken wire.
What you can safely do
Write down the full code. Check the boiler has power, then switch it off and on at its mains switch or press reset once. If the code comes back, leave it there: sensors are inside the casing.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Testing and replacing sensors means opening the casing, so this is engineer-only.
What does 1076 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1076 means the heat exchanger temperature sensor is disconnected. Likely causes include unplugged or failed sensor or a broken wire. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
PCB / electrical
233 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
The boiler's identification (code) plug or electronics are not recognised.
Likely causes
Wrong, damaged or loose code plug, or a control board fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 233 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 233 means the boiler's identification (code) plug or electronics are not recognised. Likely causes include wrong, damaged or loose code plug, or a control board fault. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
235 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
The software versions of the boiler electronics and the code plug do not match.
Likely causes
Incompatible code plug or burner control after a part has been changed.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 235 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 235 means the software versions of the boiler electronics and the code plug do not match. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
238 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
The boiler electronics have reported a fault, which the manual links to the gas valve circuit.
Likely causes
Gas valve supply or wiring problem, a failed gas valve, or a control board fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 238 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 238 means the boiler electronics have reported a fault, which the manual links to the gas valve circuit. There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
242 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
Internal system fault in the boiler electronics.
Likely causes
Loose connections on the control board or a failed control unit (the same advice is given for codes 245 to 263).
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 242 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 242 is listed in the manufacturer's table as "Internal system fault in the boiler electronics". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
360 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
System fault in the boiler electronics, linked by the manual to the code plug.
Likely causes
Wrong or damaged code plug.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 360 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 360 is listed in the manufacturer's table as "System fault in the boiler electronics, linked by the manual to the code plug". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2085 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
Internal fault in the boiler.
Likely causes
Mains supply voltage or polarity problems, or a control board fault.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 2085 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2085 is listed in the manufacturer's table as "Internal fault in the boiler". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
2908 Worcester Bosch Greenstar 4000
Engineer after one resetPCB / electricalWhat it means
System fault in the burner controller.
Likely causes
If it returns after a restart, the manual says the burner control board needs replacing.
What you can safely do
Write down the full code. Check the boiler has power and nothing else in the house has tripped, then switch the boiler off and on at its mains switch or press reset once. If the code comes back, leave the boiler off.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code returns. Board, wiring and gas valve checks mean opening the casing, so this is engineer-only.
What does 2908 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 2908 is listed in the manufacturer's table as "System fault in the burner controller". There is no homeowner fix for this. You can reset once, but do not open the casing; book a Gas Safe registered engineer and quote the full code.
Other / controls
232 Worcester Bosch Greenstar 4000
Safe checks firstOther / controlsWhat it means
The boiler is being held off by an external switching contact.
Likely causes
Something connected to the external cut-off terminal, which the manual says is not normally used in the UK.
What you can safely do
Check any external control wired to the boiler, such as a timer, frost stat or switch, is on.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code stays after you have checked the controls, or keeps coming back.
What does 232 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 232 means the boiler is being held off by an external switching contact. Reset once; if the code comes back, book a Gas Safe registered engineer and quote the full code. Do not open the casing.
1010 Worcester Bosch Greenstar 4000
Safe checks firstOther / controlsWhat it means
The boiler has lost communication with a connected control over the EMS bus.
Likely causes
Loose bus or ribbon connection, flat batteries in a room control, wrong control wiring, interference from mains cabling, or a board fault.
What you can safely do
Check your room thermostat or programmer has power or fresh batteries and is set to call for heat. For a wireless control, follow its guide to reconnect it. Then press reset once.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if the code stays after you have checked the controls, or keeps coming back.
What does 1010 mean on a Worcester Bosch Greenstar 4000 boiler?
On a Worcester Bosch Greenstar 4000, 1010 means the boiler has lost communication with a connected control over the EMS bus. Check the programmer, room thermostat batteries and any smart or wireless controls are working, then reset once. If it returns, book a Gas Safe registered engineer.
Common questions
About Worcester Bosch fault codes
7 questions
Which Worcester Bosch fault codes can I usually deal with myself?
On the Worcester Bosch ranges in our guide, only these codes have a safe homeowner check first: 1010 (lost control contact); 227, EA 227 (gas supply or flame loss); 1017 (low pressure). Reset once. Every other code needs a Gas Safe registered engineer.
Where can I find the official fault code list for my Worcester Bosch Greenstar 4000?
Worcester Bosch publishes the fault table in the installation and servicing manual for the Greenstar 4000 (6 720 891 161 (2021/10) UK/IE, pp. 71–75). CombiCert's fault code guide explains 31 codes from it. Check your data badge matches this model first, because codes differ between ranges. Write down the exact code before resetting, so the engineer can diagnose it faster.
Where can I find the official fault code list for my Worcester Bosch Greenstar i ErP?
Worcester Bosch publishes the fault table in the installation and servicing manual for the Greenstar i ErP (6 720 806 944 (2016/07), pp. 65–68). CombiCert's fault code guide explains 36 codes from it. Check your data badge matches this model first, because codes differ between ranges. Write down the exact code before resetting, so the engineer can diagnose it faster.
What is a volatile lockout on a Worcester boiler?
On older Greenstar Ri, Si and Junior models a fast-flashing blue light indicates a volatile lockout, which Worcester links to a sensor, fan or code plug issue. Switching power off and on may clear it. If it comes back, a Gas Safe engineer needs to test the components.
What does a slowly flashing blue light mean on my Worcester Greenstar Ri, Si or Junior?
On these older Greenstar models, a slow flash that is mostly off signals an ignition lockout, while a slow flash that is mostly on points to overheating. Check other gas appliances work and the condensate pipe is not frozen, then reset once as your user guide shows. If it repeats, book a Gas Safe engineer.
What does a fast flashing blue light mean on my Worcester Greenstar boiler?
On older Greenstar Ri, Si and Junior boilers, a fast-flashing blue light is a volatile lockout, which Worcester links to a sensor, fan or code plug problem. Turning the power off and on at the fused spur may clear it. If it returns, do not open the boiler; call a Gas Safe engineer.
Why is there no blue light at all on my Worcester Greenstar boiler?
No light usually means the boiler has no power. Check the fused spur switch is on, look for a tripped breaker at the consumer unit and check the boiler's own on/off control. If there is still no light, the fault is electrical inside the boiler and needs a Gas Safe engineer.
Codes by type: Greenstar 4000 (GR4700iW C combi)
7 questions
Which Worcester Bosch Greenstar 4000 fault codes relate to the flue, fan or air supply?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has 3 flue, fan or air supply codes: 214, 2910, 2915. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar 4000 fault codes mean the boiler will not light or loses its flame?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has 9 ignition or flame codes: 227, 228, 234, 269, 2916, 2920, 2923, 306, 363. For 227, first check the gas meter, prepay credit and another gas appliance, then reset once. The others need a Gas Safe registered engineer.
Which Worcester Bosch Greenstar 4000 fault codes relate to controls or other faults?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has 2 controls or other codes: 1010, 232. For 1010, first check the control has power or fresh batteries, then reset once. The other code needs a Gas Safe registered engineer.
Which Worcester Bosch Greenstar 4000 fault codes point to a control board or electrical fault?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has 7 control board or electrical codes: 2085, 233, 235, 238, 242, 2908, 360. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar 4000 fault codes point to a temperature sensor fault?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has 7 temperature sensor codes: 1021, 1037, 1065, 1073, 1074, 1075, 1076. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar 4000 fault code means the boiler is overheating?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has one overheat code, 224. 224 means a safety overheat device on the flue or heat exchanger has tripped. This code has no homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar 4000 fault codes relate to water pressure or circulation?
On the Worcester Bosch Greenstar 4000, the manufacturer's table has 2 water pressure or circulation codes: 1017, 281. For 1017, first check the pressure gauge when cold and top up if low, as your user guide shows, then reset once. The other code needs a Gas Safe registered engineer.
Codes by type: Greenstar i ErP (Greenstar 25i / 30i combi)
7 questions
Which Worcester Bosch Greenstar i ErP fault codes relate to the flue, fan or air supply?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has 5 flue, fan or air supply codes: C1 264, C6 215, C6 216, C7 214, C7 217. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar i ErP fault codes mean the boiler will not light or loses its flame?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has 5 ignition or flame codes: EA 227, EA 229, F0 238, F7 228, FA 306. For EA 227, first check the gas meter, prepay credit and another gas appliance, then reset once. The others need a Gas Safe registered engineer.
Which Worcester Bosch Greenstar i ErP fault code relates to controls or other faults?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has one controls or other code, C4 273. This is a routine pause rather than a fault, and the boiler should restart by itself after a short time. If it keeps showing, book a Gas Safe registered engineer.
Which Worcester Bosch Greenstar i ErP fault codes point to a control board or electrical fault?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has 15 control board or electrical codes, including 9U 233, A8 361, C4 237, EA 261, F0 239, F0 242, F0 258, F0 262. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar i ErP fault codes point to a temperature sensor fault?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has 6 temperature sensor codes: A7 808, CC 800, E2 222, E2 223, E2 350, E2 351. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar i ErP fault codes mean the boiler is overheating?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has 3 overheat codes: D4 341, E9 224, E9 276. None of these has a homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Which Worcester Bosch Greenstar i ErP fault code relates to water pressure or circulation?
On the Worcester Bosch Greenstar i ErP, the manufacturer's table has one water pressure or circulation code, A1 281. A1 281 means the pump appears to be stuck or running dry. This code has no homeowner fix beyond one reset. Do not open the casing; book a Gas Safe registered engineer and quote the full code.
Code still showing? Book a repair
CombiCert Ltd is a Gas Safe registered business (621146) in Telford, Shropshire. Our team of 2 to 5 engineers is led by director and Gas Safe engineer Peter Jefferson. Boiler repair from £90 + VAT, parts quoted separately.
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Explanations are written by CombiCert from the manufacturer manuals listed, which you should check for your exact model. This page is general information, not a diagnosis. If you smell gas, call 0800 111 999.