High Lockout
F.41 on Vaillant Thermocompact

Your boiler has lost its memory regarding the type of gas it uses and needs to be reconfigured by a professional to operate safely.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

(837/637 only) | No setting for the type of gas is stored | Set gas type in PCB (see page 30)

What causes this fault?

This fault occurs when the boiler's internal computer suffers a data loss and can no longer identify which fuel source it should be burning. The most common reason is that the main control board has developed an electrical defect or has been replaced without the necessary configuration codes being entered. Without this specific profile, the boiler will not fire up because it cannot guarantee a safe combustion process.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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