Boiler Fault Code On Flash On

Also written as ONFLASHON — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026

Fault code On Flash On appears across 3 boiler brands and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is struggling to accurately detect its own internal temperature, which has caused it to stop working to avoid potential damage. (As reported on Potterton Promax Heat Only)