Boiler Fault Code On On On

Also written as ONONON — same fault.

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

Fault code On On On appears across 2 boiler brands 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 struggling to communicate with its internal control system because a wire connection has likely become loose. (As reported on Potterton Promax Heat Only)