Boiler shows a fault code or has locked out — what to do
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Never repeatedly reset a boiler that keeps locking out on the same fault. Continuous resets on a gas appliance can create a dangerous situation if the underlying fault is with the burner or gas valve.
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Most likely cause & what to check
Modern boilers display fault codes when they detect a problem and shut down for safety. The code is shown on a digital display, LED pattern, or via a flashing indicator. Understanding what the code means tells you whether it is safe to reset yourself or requires an engineer.
Look up your boiler's fault code in the manual — or search "[your boiler brand] fault code [code number]" online. Common safe-to-reset faults: low water pressure (repressurise first), condensate pipe frozen (thaw it first), overheat reset (check the system is not overheating and restart).
Faults that require an engineer: gas valve failure, ignition failure (boiler lights then goes out), fan fault, flue pressure fault (typically codes relating to the fan speed sensor). These involve components you should not touch.
Press the reset button once (usually a button with a flame or reset symbol, held for 3 seconds on some models). If the boiler fires and runs normally, monitor it over the next hour. If it locks out again with the same code, the underlying fault needs repair.
If the code is for low pressure (common codes: Vaillant F22, Worcester Bosch E9, Ideal F1): check the pressure gauge — if below 1 bar, use the filling loop to repressurise to 1.5 bar, then reset.
If you are not sure what the code means or the boiler will not stay running after a reset: call a Gas Safe engineer. Provide the boiler brand, model, and fault code — a good engineer will often be able to diagnose the likely fault from these details before arriving.
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