BoilerFaultCodesViessmann 40 Fault Code
The Viessmann 40 fault code means: your boiler has detected a faulty temperature sensor in one of your heating zones, which is preventing that specific circuit from getting warm.. It appears on 9 Viessmann models in our database.
Technical description: Mixer closes - Heating circuit M2 flow temperature sensor shorted out
What causes the Viessmann 40 fault?
This fault occurs when the boiler loses connection with the sensor responsible for measuring the water temperature in your underfloor heating or radiator circuit. The most common reason is that the sensor's internal wiring has failed or short-circuited, meaning the boiler can no longer safely regulate the heat for that zone. Because the control unit doesn't know how hot the water is, it closes the mixing valve as a safety precaution to prevent overheating.
This fault often becomes apparent during the first cold spell of autumn when the heating is switched on after being idle all summer, putting strain on aging electronic sensors.
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 40 fault code
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Source
Viessmann 40 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