In short
A boiler that keeps cutting out is usually protecting itself. Common causes are low pressure, a frozen or blocked condensate pipe, a thermostat problem, a gas supply issue, poor water flow or a failing part. Check the pressure, condensate and thermostat, note any fault code and reset once. If it keeps happening, call a Gas Safe engineer.
Is it cutting out, or cycling?
There are two different problems that people describe as cutting out:
- Locking out: the boiler stops and shows a fault code or light, and will not restart until it is reset. It has detected something wrong.
- Short cycling: the boiler fires for a few minutes, stops, then starts again, over and over, without a fault code. That often points to controls, flow or a boiler that is too big for the heating load.
Common causes of a boiler cutting out
Low pressure
Most sealed boilers will not run if the pressure drops too low. See why is my boiler losing pressure?
A frozen or blocked condensate pipe
In cold weather this is one of the most common reasons a condensing boiler locks out. See frozen condensate pipe.
Thermostat and controls
A room thermostat with flat batteries, a wireless thermostat that has lost its signal, or a thermostat near a heat source can switch the boiler off early.
Gas supply
A prepayment meter running out of credit, or a problem with the gas supply, will stop the boiler lighting. Check whether your hob or other gas appliances work.
Poor water flow or overheating
Sludge, a failing pump or closed valves can make the boiler overheat and shut down. Cold-bottomed radiators and noise are clues.
Ignition, flame sensing or the fan
If the boiler lights then cuts out after a few seconds, the cause may be the flame sensor, the gas valve, the fan or a blocked flue. These are engineer-only.
Safe checks you can do
Try these in order. They do not involve gas parts. If the boiler keeps cutting out after one reset, stop and call a Gas Safe engineer.
- Read the pressure gauge when cold and compare it with your user guide (often about 1 to 1.5 bar). Top up only as the guide shows; see boiler losing pressure.
- In freezing weather, check the condensate pipe outside. Thaw with warm, not boiling, water if it is frozen. See frozen condensate pipe.
- Check the room thermostat: batteries, set temperature and, on wireless models, that it still has a signal.
- Check other gas appliances work and that any prepayment meter has credit. If nothing gas-powered works, the supply is the issue.
- Note the fault code, then reset once following the user guide. Look the code up in our fault code search.
- If it cuts out again, leave the boiler off and book a Gas Safe engineer. Do not keep resetting and do not open the casing.
What CombiCert would check
We read any fault code, check pressure, the condensate pipe, flue terminal, gas supply and basic controls, then test further if those are clear. The goal is to find why the boiler is protecting itself, not to keep resetting it. Repairs from £90 + VAT.
When to call a Gas Safe engineer
If the boiler locks out again after one reset, stop resetting it. Repeated resets do not cure the cause and, with some faults, can make things worse. Book a boiler repair and tell us the code. If you smell gas, or there are signs of carbon monoxide such as headaches, sooty marks or a lazy yellow flame, turn the appliance off, open windows and call 0800 111 999.
Need a hand? CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
Boiler repairs and services from £90 + VAT. Commercial work by quote.
See Boiler repairs in Telford, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
We cover Telford and nearby, including Wellington, Madeley, Dawley, Oakengates, Shifnal, Newport, Ironbridge and Shrewsbury.
Sources
Frequently asked questions
Why does my boiler fire up then cut out after a few seconds?
This usually means the boiler has lit but cannot confirm the flame, or has detected a fault. Causes include the flame sensor, gas pressure, a blocked condensate or flue, or a fan fault. Note the code, check other gas appliances work and the condensate is not frozen, reset once, then book a Gas Safe engineer.
Why does my boiler keep switching on and off every few minutes?
That is short cycling. Common causes are a thermostat in the wrong place, a boiler set too high or too big for the heating load, poor water flow from sludge or a pump fault, or closed radiator valves. It wastes gas and adds wear, so ask a heating engineer to check the controls, flow and settings.
Is it safe to keep resetting my boiler?
Reset it once. If it locks out again, leave it off and call a Gas Safe registered engineer. Repeated resets do not cure the cause, and with some faults they can make things worse. Note any fault code on the display, because it helps the engineer find the problem quickly.
Can low pressure make my boiler cut out?
Yes. Most sealed combi and system boilers have a low pressure safety switch and will stop working if the pressure drops too far. Check the gauge with the boiler cold and top up as your user guide explains. If it keeps dropping, there is a leak or a faulty part that needs finding.
This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.