Your boiler is operating normally and is currently heating your home or water.

Illuminates when the pilot flame lights followed by the main burner; extinguishes if the Flame Failure neon illuminates or if the Flue Safety Thermostat is activated.

What causes this fault?

This is not actually a fault, but a status indicator confirming that the boiler has successfully ignited. It shows that the internal gas valve has opened and the burner is currently producing the heat required for your central heating or hot water. If this light is on, it means the system is functioning exactly as it should be.