BoilerFaultCodesViessmann 20 Fault Code
The Viessmann 20 fault code means: your boiler has detected an electrical fault with a sensor that monitors the water temperature flowing into your heating system, leading it to shut down for safety.. It appears on 9 Viessmann models in our database.
Technical description: Controls without flow temperature sensor (low loss header): System flow temperature sensor shorted out
What causes the Viessmann 20 fault?
This fault occurs when the electrical signal from the temperature sensor is interrupted or fails to send back the correct information. The most common reason is a physical failure of the internal sensor components or damage to the wiring that connects the sensor to the boiler's main control board. If the boiler cannot accurately measure how hot the water is, it shuts down to prevent the system from 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 20 fault code
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Source
Viessmann 20 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