Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use insulation mat as necessary)
Disconnect plug connector (marked JP11) from PCB
Remove the microswitch from the top of the diverter valve

Measurement:

Check resistance across connections 3 & 4 (Brown & Blue wires)
Expect short circuit.
Now depress the microswitch.
Expect open circuit.
Note:
Care should be taken that probe tips do not touch each other when taking measurement as a false reading will be given.