Your boiler is currently running a maintenance program to help fill or drain the system, meaning it has temporarily paused its normal heating and hot water functions.

Filling/draining programme: The preference changeover valve moves to the centre position. The burners and pump are switched off.

What causes this fault?

This code appears when the boiler's internal software has been manually or automatically put into a specialized service mode for maintenance. It physically shifts the internal valves to a central position to allow water to flow freely through the system for filling or draining. Because the boiler is focusing entirely on moving water, it temporarily stops the burner from firing to prevent overheating.