BoilerFaultCodesViessmann F9 Fault Code
The Viessmann F9 fault code means: your boiler's fan is not spinning fast enough to safely start the burner, causing the system to shut down for safety.. It appears on 9 Viessmann models in our database.
Technical description: Burner in fault state. Fan speed too low at burner start. Cause: Check the fan, check the fan cables and supply, check the fan control. Press E.
What causes the Viessmann F9 fault?
This fault occurs when the boiler's internal computer tries to start the heating process but detects that the fan isn't reaching the necessary speed to clear out exhaust gases. The most common reasons are a build-up of debris causing the fan to stick, a failure in the fan's motor, or a loose electrical connection between the fan and the main control board.
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 F9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann F9 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