Your boiler is perfectly healthy and is simply letting you know that it is currently preparing or providing heat to your radiators or hot water cylinder.

What causes this fault?

This status occurs when the boiler receives a signal from your thermostat or timer that the house needs heating or the hot water tank needs warming. It indicates that the internal controls have successfully registered this request and the boiler is now initiating its firing sequence. It is the standard starting point for normal operation rather than a mechanical failure.