Boiler Fault Code 5 03

Also written as 503 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026

Fault code 5 03 appears across 1 boiler brand and 3 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 a flame when it should be turned off, which is a significant safety precaution that prevents the unit from starting until it is inspected. (As reported on Ariston Alteas One)