Your boiler is currently transitioning between modes or performing a safety check while the burner gets ready to fire up.

Burner starting

What causes this fault?

This is a normal operational status rather than a fault, occurring as the boiler's internal computer synchronises the gas valve and fan speeds before ignition. It physically represents the moment the boiler is double-checking all safety sensors are satisfied before it allows the burner to light. The code appears briefly during the pre-purge phase to ensure the combustion chamber is clear and safe to operate.