Why does my boiler keep cutting out?

This is one of the most searched heating faults and one of the least well distinguished, because the same phrase describes two quite different behaviours with different causes. Working out which you have takes a minute and decides everything after it.

Safety

Step by step

1. Establish which behaviour you have.Does it stop and stay off until you press reset? That is a lockout. Does it fire, run briefly, stop and start again by itself? That is short-cycling. They have almost nothing in common beyond the description.
2. Note the fault code before resetting.Photograph the display. Resetting clears it, and the code is the most useful single piece of information for anyone diagnosing it afterwards — including you.
3. Check the pressure gauge first.Should sit in the marked band, typically 1 to 1.5 bar when cold. Low pressure is the commonest cause of a lockout and is repressurised in a couple of minutes. If it keeps dropping, there is a leak.
4. Check the condensate pipe in cold weather.A frozen condensate pipe shuts a boiler down and is the most common winter cause. Gurgling as it tries to fire is the tell. Warm the external pipe with hot tap water and reset.
5. Check the gas is actually on.Other gas appliances working, and any prepayment meter in credit. A boiler with no gas locks out on ignition failure, and it is a surprisingly common answer.
6. Check the thermostat and timer.Flat batteries in a wireless thermostat, a lost pairing, or a schedule that has drifted after a clock change. The boiler is behaving correctly and being told to stop.
7. Consider overheating for short-cycling.If it fires and cuts out within a minute or two repeatedly, the boiler is reaching temperature far too fast. That usually means water is not circulating — a closed valve, a stuck pump, sludge, or a blocked heat exchanger.
8. Check the radiator valves are open.If most radiators are turned off, there is nowhere for the heat to go and the boiler overheats and cuts out. Open several fully and see whether the behaviour changes. This is a genuinely common cause after someone turns rooms down.
9. Look for cold-at-the-bottom radiators.Sludge restricts circulation, which makes a boiler cycle. Radiators hot at the top and cold at the bottom are the symptom, and a power flush plus a magnetic filter is the fix.
10. Check for an oversized boiler.A boiler much larger than the house needs will short-cycle in mild weather because it satisfies demand faster than it can modulate down. Nothing is broken; it is a sizing issue an engineer can partly manage with settings.
11. Reset once, and only once.One reset to see whether the fault recurs is reasonable. Repeated resetting overrides a protection that has operated for a reason, and the underlying fault continues.
12. Have the filter cleaned and the boiler serviced.A clogged magnetic filter restricts flow and causes cycling. An annual service catches it, and many warranties require one anyway.
13. Call an engineer with the detail.The fault code, whether it locks out or cycles, how often, what the pressure reads and what you have already checked. That turns a diagnostic visit into a repair visit.

Tips

• Write the boiler's normal cold pressure on a label beside the gauge. Knowing what normal looks like makes an abnormal reading obvious at a glance.
• If it cuts out only when the heating and hot water run together, that points at flow or a diverter valve rather than the burner.
• A boiler that has cut out several times in a week is telling you something. It is far cheaper to have it looked at than to have it fail entirely in December.

Common mistakes

• Resetting repeatedly to keep it running — A lockout is a protection operating. Overriding it leaves the underlying fault — low pressure, no flow, ignition failure — running unaddressed.
• Clearing the fault code before noting it — The code is the most useful diagnostic information available, and resetting erases it. Photograph the display first.

If it doesn't work

Questions people ask