Your boiler is currently set to a specific maintenance mode used by engineers to replace internal parts, rather than being in a fault state.

To move the diverter valve to mid-position for replacement

What causes this fault?

This code appears because the boiler has been manually put into a specialized service mode by a person pressing specific buttons on the control panel. It is not caused by a mechanical failure, but rather by the system being told to move the internal diverter valve to a central position so it can be safely swapped or repaired. The boiler will remain in this state and will not provide heating or hot water until the mode is exited.