High Lockout
F.41 on Vaillant ecoMAX 635

Your boiler has lost its electronic connection to the gas control valve, meaning it cannot safely release fuel to start the flame.

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

Gas valve not detected

What causes this fault?

This fault occurs when the boiler's main control board loses communication with the gas valve, preventing the system from regulating the fuel supply. The most common reason is a loose or damaged wiring connection between these two components, though it can also be caused by a complete electronic failure within the gas valve itself.

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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Related fault codes

F.22View →F.61View →

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