BoilerFaultCodesViessmann 9B Fault Code
The Viessmann 9B fault code means: your boiler has lost connection with its internal hot water storage sensor and cannot currently heat your water effectively or safely.. It appears on 7 Viessmann models 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 can no longer communicate with the sensor responsible for monitoring the temperature of your hot water. The most common cause is a physical failure of the internal sensor probe or a loose connection in the wiring harness that links the sensor to the main computer. Without this signal, the boiler cannot safely determine how hot the water is, so it stops heating the cylinder to prevent overheating.
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 9B fault code
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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 27 April 2026