Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Turn Hot Water Tap On
Remove Plastic covers from the rear of the Control Box
Turn OFF DHW Inlet Isolating Valve
Disconnect connector marked X11 from Main Switchboard
There are 7 black leads
The testing will be carried out at the two end wires (1 and 2)
Note:
Numbers run from left to right.

Measurement:

Open DHW Isolating Valve and allow Diverter Valve to operate.
Measure resistance across group of 2 black leads at the disconnected connector X11.
Resistance should be short circuit.
Close DHW Isolating Valve.
Measure resistance across the group of two black leads at the disconnected connector X11.
Resistance should be open circuit.