Emergency
F.355 on Viessmann Vitodens 100-W WB1A

Your boiler’s internal control system has detected a flame signal when there shouldn't be one, which prevents the boiler from starting safely.

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.

What the manual says

Manufacturer manual

Analogue signal reference check: Flame signal is already present at burner start

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1a installation & service manual, page 88.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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