BoilerFaultCodesViessmann 50:3 Fault Code
Also written as 503 — same fault.
The Viessmann 50:3 fault code means: your boiler's outside temperature sensor has stopped communicating with the system, which may cause your heating to run less efficiently.. It appears on 5 Viessmann models in our database.
Technical description: Weather-compensated control units
What causes the Viessmann 50:3 fault?
This fault occurs when the boiler loses communication with the sensor mounted on your external wall. The most common reason is that moisture or corrosion has damaged the sensor's internal circuitry, or the electrical wiring connecting it to the boiler has become loose or broken. Because the boiler no longer knows how cold it is outside, it cannot automatically adjust its temperature to run at maximum efficiency.
This fault is more frequently reported during autumn and winter when heavy rain or freezing temperatures can expose existing damage or poor weather-sealing in the external sensor.
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 50:3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 50:3 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