BoilerFaultCodes Red light F ON High
Boiler Fault Code Red light F ON
Also written as REDLIGHTFON — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code Red light F ON appears across 1 boiler brand and 1 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 safety issue and has automatically switched itself off to prevent damage or dangerous operation. (As reported on Saunier Duval SD 30e)
Pick your boiler brand for the Red light F ON fix
Source
Information for the Red light F ON 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