BoilerFaultCodesViessmann b4 Fault Code
Also written as B4 — same fault.
The Viessmann b4 fault code means: your boiler has not been fully configured for operation or there is a communication problem between the internal controls and the burner assembly.. It appears on 8 Viessmann models in our database.
Technical description: Installation in unfinished state - Fault in burner / Ignition valve
What causes the Viessmann b4 fault?
This fault occurs when the boiler's internal computer cannot successfully communicate with the burner system or hasn't been properly programmed to start. It is often caused by a loose connection, a faulty ignition lead, or a breakdown in the electronic control board that manages the gas flow. Effectively, the boiler has safety-locked because it cannot verify that it is safe to ignite the flame.
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 b4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann b4 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