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
Disconnect plug connector (marked ST16) from PCB

Measurement:

Check resistance across connections 19 & 20 (pink wires).
Note:
Care should be taken that probe tips do not touch each other when taking measurement as a false reading will be given. Also ensure that other end of leads are connected to the diverter valve microswitch.