BoilerFaultCodesViessmann 09 Fault Code
The Viessmann 09 fault code means: your boiler has detected a sequence error or a flame signal where there shouldn't be one, causing the burner to shut down completely for safety.. It appears on 7 Viessmann models in our database.
Technical description: Burner control switches to lockout
What causes the Viessmann 09 fault?
This fault occurs when the boiler detects a persistent flame signal even after the burner should have switched off, or if the internal electrical checks fail during start-up. The most common reason is a build-up of carbon or moisture on the ionisation probe, which tricks the system into thinking a fire is still burning. In some cases, a fault within the main control board or damaged ignition wiring prevents the boiler from safely managing the combustion process.
This fault can be more frequent in autumn or winter when increased condensation or dampness in the air can affect sensitive electrical components and 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 09 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 09 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