Boiler Fault Code Light ON

Also written as LIGHTON — same fault.

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

Fault code Light ON appears across 1 boiler brand and 6 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is switched on and ready to work, but it isn't responding when you turn up the heating or hot water, usually because an external controller or a valve is failing to tell the boiler to start. (As reported on Worcester Bosch Greenstar 12Ri)