Boiler Fault Code Lights: On Flash On

Also written as LIGHTSONFLASHON — same fault.

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

Fault code Lights: On Flash 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 detected a faulty temperature sensor, which means it cannot accurately measure how hot the water is and has stopped running as a safety precaution. (As reported on Baxi Solo 3 40 PF)