BoilerFaultCodes 303 High
Boiler Fault Code 303
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 303 appears across 2 boiler brands and 19 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has detected an internal temperature sensor failure, meaning it cannot properly monitor its own performance and has safely shut down to prevent damage. (As reported on Navien NCM 1130HH)
Pick your boiler brand for the 303 fix
Source
Information for the 303 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 31 August 2026