BoilerFaultCodes 50:3 Low
Boiler Fault Code 50:3
Also written as 503 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 50:3 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler's outside temperature sensor has stopped communicating with the system, which may cause your heating to run less efficiently. (As reported on Viessmann Vitodens 300-W)
Pick your boiler brand for the 50:3 fix
Source
Information for the 50:3 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026