Your boiler is currently firing up and running as it should to provide heating or hot water, so this is a status message rather than a fault.

burner on

What causes this fault?

This code appears when the boiler's internal computer has successfully ignited the burner to meet a demand for heat. It signifies that the gas valve has opened and a flame is present, confirming that the system is operating exactly as intended. There is no physical fault or blockage causing this display; it is simply the boiler communicating its current activity.