BoilerFaultCodesViessmann 9C Fault Code
The Viessmann 9C fault code means: your boiler is unable to read the temperature of your hot water cylinder because the connection to the sensor has been lost or damaged.. It appears on 7 Viessmann models in our database.
Technical description: Control mode: Cylinder temperature sensor lead break
What causes the Viessmann 9C fault?
This fault occurs when the boiler loses its electrical connection to the temperature sensor attached to your hot water tank. The most common reason is that the thin wiring has become loose, corroded, or accidentally damaged, preventing the boiler from knowing if your water is hot enough. Because the control board can no longer monitor the safety of the cylinder, it stops providing hot water 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 9C fault code
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Source
Viessmann 9C 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