BoilerFaultCodesViessmann b4 Fault Code
Also written as B4 — same fault.
The Viessmann b4 fault code means: your boiler has lost the connection to its outdoor temperature sensor and is assuming a fixed temperature of 0°c to keep your heating running safely.. It appears on 4 Viessmann models in our database.
Technical description: Regulates as if the outside temperature was 0 °C
What causes the Viessmann b4 fault?
This fault occurs when the boiler's internal computer is unable to process the data it receives from its electronic sensors. The most common reason is a component failure on the main circuit board, which prevents the system from accurately measuring how the boiler is burning gas.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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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.
- Viessmann Vitodens 050-W manual (PDF)
- Viessmann Vitodens 222-F manual (PDF)
- Viessmann Vitodens 242-F B2UA manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026