BoilerFaultCodesWorcester Bosch Light ON Fault Code
Also written as LIGHTON — same fault.
The Worcester Bosch Light ON fault code means: your boiler is switched on and ready to work, but it isn't responding when you turn up the heating or hot water, usually because an external controller or a valve is failing to tell the boiler to start.. It appears on 6 Worcester Bosch models in our database.
Technical description: Boiler ON but not operating during demand. Possible solutions/checks: Live demand at ST2 terminal L - is there a demand?, Manual bypass is open. If CH is working check HW controls, If HW is working check CH controls (i.e. external diverter valve, frost stat, room stat/programmer. Otherwise replace board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Worcester Bosch models that show the Light ON fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Light ON fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
- Worcester Bosch 26 CDi Xtra manual (PDF)
- Worcester Bosch 35 RSF manual (PDF)
- Worcester Bosch Greenstar 12Ri manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026