Your boiler is not broken; it is simply in a special service mode used by engineers when they are preparing to replace internal parts.

To move the diverter valve to mid-position for replacement

What causes this fault?

This code appears when someone has manually activated the diverter valve service function, usually by pressing a specific combination of buttons. It is a programmed setting that moves internal components into a neutral position to allow an engineer to safely drain the unit and replace parts. It is not caused by a mechanical failure but by the boiler being placed into a pre-service state.