BoilerFaultCodes 3 03 High
Boiler Fault Code 3 03
Also written as 303 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 3 03 appears across 1 boiler brand and 14 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler's internal computer is experiencing a communication fault, preventing it from safely starting or controlling the heating process. (As reported on Ariston Clas HE Evo)
Pick your boiler brand for the 3 03 fix
Source
Information for the 3 03 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