BoilerFaultCodesViessmann b5 Fault Code
Also written as B5 — same fault.
The Viessmann b5 fault code means: your boiler's main internal computer has encountered a communication error and cannot control the heating process correctly.. It appears on 6 Viessmann models in our database.
Technical description: Control mode - Internal error
What causes the Viessmann b5 fault?
This fault occurs when the boiler's central brain suffers a hardware failure or a critical data mismatch within its internal circuits. The most common reason is a breakdown of the communication between the main circuit board and the specialized coding plug that stores the boiler's specific operating instructions.
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 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 Compact manual (PDF)
- Viessmann Vitodens 242-F B2UA manual (PDF)
- Viessmann Vitodens 242-F FB2B manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026