Boiler Fault Code On / Off

Also written as ONOFF — same fault.

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

Fault code On / Off appears across 2 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 providing hot water as normal, but there is a communication issue preventing the heating from turning on when you use your thermostat. (As reported on Worcester Bosch 28i Junior)