BoilerFaultCodesViessmann F3 Fault Code
The Viessmann F3 fault code means: your boiler has shut down because it thinks it can see a flame before it has even started the ignition process.. It appears on 9 Viessmann models in our database.
Technical description: Burner in fault state. The flame signal is already present at burner start. Cause: Check the ionisation electrode and leads. Press E.
What causes the Viessmann F3 fault?
This fault occurs when the boiler's internal computer detects a flame signal at a time when there shouldn't be one, such as during the initial safety check before ignition. The most common cause is a faulty or soot-covered ionisation electrode that is sending a false electrical signal, or moisture trapped within the burner's wiring. Because the boiler cannot verify that the burner is truly off, it shuts down as a safety precaution to prevent a buildup of unburnt gas.
This fault can be more frequent during periods of high humidity or heavy rain, as moisture entering the air intake can interfere with the sensitivity of the ionisation probes.
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 F3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann F3 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