Emergency Lockout
F8 on Viessmann Vitodens 333

Your boiler has detected a potential issue with the valve that controls the gas supply, meaning it isn't shutting off correctly for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description

Burner in fault state. Fuel valve closes too late. Cause: Check gas combination valve. Check both control paths. Press E.

What causes this fault?

This fault occurs when the boiler's internal computer detects that gas is still flowing for a few seconds after the burner was supposed to switch off. The most common reason is a mechanical failure within the gas valve itself, where it becomes 'sticky' and fails to snap shut instantly, or a fault in the electronic control board that manages the safety sequence.

Engineer required

This is a emergency 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

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