Your boiler is not showing an error; it is simply displaying a status message to let you know that the burner is currently active and heating your water.

burner on

What causes this fault?

This code appears when the boiler is functioning correctly and the internal burner has successfully lit to heat your home or hot water. It is triggered by a demand from your thermostat or a hot tap, signaling that the gas valve has opened and the ignition process is complete. There is no fault to fix as this is simply the boiler confirming it is currently burning fuel to provide heat.