BoilerFaultCodesWorcester Bosch Light on Fault Code
Also written as LIGHTON — same fault.
The Worcester Bosch Light on fault code means: your boiler is receiving power and trying to work, but it has failed to start the burner to provide heat or hot water.. It appears on 12 Worcester Bosch models in our database.
Technical description: Appliance on but not operating during demand
What causes the Worcester Bosch Light on fault?
This fault occurs when the boiler is receiving an electrical signal to start, but the internal sequence to light the flame is interrupted. It is often caused by a lack of spark from the ignition electrode or a failure in the internal fan which clears waste gases. In some cases, the gas valve may not be opening correctly to allow fuel into the burner, preventing the boiler from firing up.
DIY fix possible
Check if your thermostat is set higher than the current room temperatureEnsure your external programmer or timer is set to 'On'Check that the gas supply to the property is turned onPress the reset button on the front of the boiler
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Gas Safe Register →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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026