BoilerFaultCodesViessmann F8 Fault Code
The Viessmann F8 fault code means: your boiler has detected a problem with the gas valve not closing correctly, so it has shut down the burner for safety.. It appears on 9 Viessmann models in our database.
Technical description: Burner in fault position. Fuel valve closes too late. Check the gas train and both control cables.
What causes the Viessmann F8 fault?
This fault occurs when the boiler's internal computer detects that the gas valve is taking too long to shut off or is failing to close completely after a heating cycle. The most common reason is a mechanical failure within the valve assembly itself or a communication error in the wiring harness that prevents the signal to close from being received instantly. For safety reasons, the boiler identifies this as a risk and prevents the burner from firing again until the issue is resolved.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the F8 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann F8 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026