Boiler Fault Code Lights: On Off On

Also written as LIGHTSONOFFON — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026

Fault code Lights: On Off On 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 failed to light the flame, so it has safely shut itself down to prevent a gas build-up. (As reported on Potterton Promax HE Store)