Boiler Fault Code On Off Flash

Also written as ONOFFFLASH — same fault.

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

Fault code On Off Flash appears across 2 boiler brands 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 that the electricity supply wires are connected the wrong way around, which prevents it from operating safely. (As reported on Potterton Promax Heat Only)