BoilerFaultCodesViessmann b8 Fault Code
Also written as B8 — same fault.
The Viessmann b8 fault code means: your boiler has detected a broken connection or a fault with the sensor that monitors the temperature of the exhaust gases, causing the system to stop working for safety.. It appears on 9 Viessmann models in our database.
Technical description: Burner blocked: Flue gas temperature sensor break
What causes the Viessmann b8 fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors the heat of the exhaust gases. The most common reason is that the sensor has physically failed or developed a break in the internal wiring, though it can also be triggered if the sensor has become dislodged from its mounting clip. For safety, the boiler shuts down immediately to ensure the exhaust system does not overheat.
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 b8 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann b8 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