BoilerFaultCodesViessmann 50:1 Fault Code
Also written as 501 — same fault.
The Viessmann 50:1 fault code means: your boiler has detected a short circuit in the sensor that monitors your hot water temperature, meaning it can no longer accurately heat your tap water.. It appears on 5 Viessmann models in our database.
Technical description: Constant temperature control units
What causes the Viessmann 50:1 fault?
This fault occurs when the electrical signal from the hot water sensor becomes constant, usually because the internal wiring of the sensor has failed or become damaged. This prevents the boiler from knowing the temperature of the water in your cylinder, causing it to stop heating your taps to prevent overheating. The most common reason is a component failure within the probe itself or water ingress affecting the electrical connections.
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 50:1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 50:1 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