Your boiler has been manually set to a maintenance mode for filling or draining the system, meaning it is currently paused and will not provide heating or hot water.

Filling mode: The prioritising diverter valve is moved to the mid-position. The burner and pump are switched off (to fill or drain the product).

What causes this fault?

This code appears because the boiler has been manually put into a specific service mode by a person interacting with the control panel. It is triggered when someone is intending to fill, drain, or bleed the system, which causes the boiler to pause its normal operation for safety. The boiler hasn't broken; it is simply waiting in a 'standby' state while maintenance tasks are performed.