BoilerFaultCodesViessmann b5 Fault Code
Also written as B5 — same fault.
The Viessmann b5 fault code means: your boiler has experienced an internal communication error or processor glitch that is preventing it from running properly.. It appears on 4 Viessmann models in our database.
Technical description: Control mode: Internal fault
What causes the Viessmann b5 fault?
This fault occurs when the boiler's internal computer experiences a breakdown in communication with its own components or sensors. It is usually caused by a hardware failure within the main control board or a loose connection in the internal wiring, meaning the boiler can no longer verify it is safe to operate.
DIY fix possible
Locate the power switch or fused spur for the boilerTurn the power off, wait for 30 seconds, and turn it back onPress the 'Reset' button on the boiler control panel once
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Viessmann models that show the b5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann b5 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 100-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