Your boiler is working correctly and is simply indicating that the internal burner is lit and ready to provide heating or hot water.

Indicates that the pilot valve is held open, the thermoelectric valve and spark unit are de-energised, and the system is operating or ready.

What causes this fault?

This indicator light means that your boiler has successfully sparked and established a flame to heat your water. It shows that the internal gas valve is open and the system is functioning exactly as it should be. There is no fault present, and the light is simply confirming that the burner is currently active.