BoilerFaultCodesViessmann 51 Fault Code
The Viessmann 51 fault code means: your boiler is unable to provide hot water because of a faulty internal sensor that has stopped communicating correctly.. It appears on 8 Viessmann models in our database.
Technical description: No DHW heating - Cylinder temperature sensor 2/outlet sensor shorted out
What causes the Viessmann 51 fault?
This fault occurs when the internal electronic probe responsible for monitoring the hot water temperature develops an electrical short circuit. The boiler’s control board can no longer read the water temperature safely, so it disables the hot water function to prevent overheating. This is typically caused by natural wear and tear of the sensor or moisture ingress damaging the electrical connections over time.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Viessmann models that show the 51 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 51 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