BoilerFaultCodesBoiler keeps cutting out or locking out?
There's a difference between "cycling" (firing up briefly and stopping, normal in mild weather) and "locking out" (firing up and shutting down with a fault code that needs a reset). If yours is doing the latter, something is triggering a safety system. These are the six causes we see most often.
Don't keep hitting reset
Repeated resets mask the real problem and on some boilers will eventually damage the PCB. If it locks out twice, stop resetting and diagnose — starting with the fault code on the display.
The six usual causes
Low water pressure
Most common. Pressure below ~0.8 bar triggers lockout on most modern boilers. Top up using the filling loop and see if the fault returns. If it does, you have a leak somewhere — see our losing pressure guide.
Typical codes: Worcester E9, Vaillant F22, Ideal F1
Frozen or blocked condensate pipe
In winter, the plastic pipe that drains condensate from the boiler freezes. Boiler detects the blocked flue and shuts down. Pour warm water on the pipe outside to thaw it.
Typical codes: Worcester EA, Vaillant F28/F29, Ideal L2
Ignition failure
Boiler tries to ignite, fails, locks out. Causes: gas supply issue (check other gas appliances), dirty or worn electrode, failed ignition lead, or frozen condensate.
Typical codes: Worcester EA/E9, Vaillant F28, Ideal L2
Flame loss
Boiler fires then the flame goes out mid-cycle. Usually a failing flame detection electrode or a gas valve issue. Gas Safe engineer job.
Typical codes: Vaillant F27, Ideal F2, Worcester EA
Overheating (no flow through heat exchanger)
Blocked heat exchanger (sludge/limescale) or failing pump means water isn't circulating. Boiler heats up, triggers overheat sensor, shuts down. Usually accompanied by kettling noise.
Typical codes: Worcester E9/E1, Vaillant F20, Ideal L1
Faulty PCB
Rarest but nastiest. The circuit board misreads a sensor and shuts down for no real reason. Fault codes may appear to be random or change each time. Cost: £250–£500 fitted.
Typical codes: Varies — often F9 / F62 / E168
Before the engineer visit — note these down
Having this info ready can save 20 minutes of diagnostic time and £40 on the call-out.
- › The exact fault code shown before resetting
- › How often it locks out (every 5 mins? Once a day?)
- › When it started (after a cold snap? After a service?)
- › Pressure reading when cold
- › Any unusual noises before the lockout