BoilerFaultCodesWorcester Bosch On / Off Fault Code
Also written as ONOFF — same fault.
The Worcester Bosch On / Off fault code means: 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.. It appears on 3 Worcester Bosch models in our database.
Technical description: Boiler not operating during central heating demand (HW ok)
What causes the Worcester Bosch On / Off fault?
This issue occurs when the link between your heating controls and the boiler is broken, meaning the boiler doesn't 'hear' the signal to turn on. The most common reason is a pair of flat batteries in your wall thermostat or a technical failure in the wireless receiver box attached to the boiler.
This fault is frequently discovered during the first cold snap of autumn when homeowners try to turn their heating on for the first time in several months.
DIY fix possible
Check that the batteries in your wall thermostat or room programmer haven't run outEnsure the heating timer is set to 'ON' or 'AUTO' and the thermostat is turned up higher than the current room temperatureCheck if any wireless receiver attached to the boiler has a power light or is showing a signal connection
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Gas Safe Register →Worcester Bosch models that show the On / Off fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch On / Off fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026