Emergency
A0 on Viessmann Vitodens 100-W

Your boiler has detected a flame before it has actually started up, which is a safety mechanism preventing it from firing when it shouldn't.

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 blocked, flame signal is present at burner start

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

F4View →F5View →

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