Boiler Fault Code On On On On

Also written as ONONONON — same fault.

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

Fault code On On On On appears across 1 boiler brand and 1 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 water is failing to drain away properly and has shut itself down to prevent internal flooding. (As reported on Potterton Promax FSB 30HE)