BoilerFaultCodesViessmann b9 Fault Code
Also written as B9 — same fault.
The Viessmann b9 fault code means: your boiler has likely been left in a testing mode used during installation or there is a fault with the gas ignition valve.. It appears on 1 Viessmann model in our database.
Technical description: Installation in unfinished state - Fault in burner / Ignition valve
What causes the Viessmann b9 fault?
This fault typically occurs because the boiler's software thinks the setup process hasn't been finished or it has detected a physical failure in the gas delivery system. The most common reasons are a malfunctioning gas valve that cannot control the flow of fuel correctly or an ignition electrode that is failing to spark properly. In some cases, it may simply be that an engineer forgot to deactivate a specific 'commissioning mode' after a repair or installation.
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 b9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann b9 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