Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4

Observation:

Check selector switch control knob is fitted correctly, the control knob should move between Off, DHW only and DHW & CH with a click when it fits into each position. There should be a spring action when moving to the "reset" position.
Note:
The spring on the control knob is not fixed, take care if control knob is removed for inspection.