BoilerFaultCodesViessmann 9B Fault Code
The Viessmann 9B fault code means: your boiler has lost communication with one of its internal heat sensors, meaning it cannot monitor temperatures accurately to operate safely.. It appears on 1 Viessmann model in our database.
Technical description: Control mode: Sensor S3 lead break
What causes the Viessmann 9B fault?
This fault occurs when the boiler's control board loses its electrical connection to a specific internal temperature sensor. The most common cause is a failure of the sensor itself or loose, damaged wiring that prevents the boiler from safely monitoring the water temperature. Without this data, the system shuts down to prevent overheating.
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 9B fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 9B 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 7 July 2026