Why does my boiler keep turning off or locking out?

Modern boilers are designed to lock out rather than keep running through a fault, which is a safety feature, not a malfunction in itself. The code shown on the display is the boiler telling you which safety check it failed — the difficulty is that every manufacturer uses its own system, so a code that means one thing on a Worcester Bosch means something different on a Vaillant, Baxi or Ideal. Before calling anyone, photograph the exact code and note whether it happened once or is repeating; our own plumbing and heating engineers ask for exactly that on every call — that alone saves an engineer time on site.

Low pressure lockouts

The single most common reason a boiler locks out. If the gauge is reading below about 1 bar, that is very likely your answer — our piece on why boilers lose pressure covers the difference between a slow, normal drop and one that means a leak somewhere in the system.

Ignition faults

The boiler tries to light and fails, usually more than once before it locks out rather than keep trying. This can be as simple as an interruption to the gas supply, or point to a fault with the ignition electrode or the flame sensor itself. It is not something to keep resetting repeatedly — a boiler that will not ignite after two attempts needs an engineer rather than a third attempt.

Blocked condensate pipe faults

Very common in cold weather specifically, and easy to mistake for something more serious. Condensing boilers produce acidic waste water that drains away through a plastic pipe, often routed outside — and in freezing conditions the standing water in that pipe can freeze solid and trigger a lockout. This is common enough, and usually simple enough to fix yourself, that it has its own explanation — see our guide on what to do about a frozen condensate pipe.

Fan or pressure switch faults

The boiler cannot confirm the fan is clearing exhaust gases properly, and locks out rather than risk running unsafely. This can be the fan itself wearing out, or something simpler like a blocked flue terminal outside the property — worth a quick visual check before assuming the worst.

What to do before calling

Check the pressure gauge first, since that alone explains a large share of lockouts. If the pressure is normal, try one reset following the instructions printed on the boiler — but stop there. Repeatedly resetting a boiler that keeps locking out with the same code does not fix the underlying fault and can occasionally make it worse. If it is affecting hot water as well as heating, our piece on no hot water but the boiler is working may point to a related but separate cause. Anything beyond a single reset is a job for boiler repair, and a boiler that will not stay on at all in cold weather is worth treating as an emergency rather than waiting for a routine slot.

Outside our coverage area, other London Gas Safe engineers such as Ninja Plumbers in Fulham deal with the same fault codes day to day.