Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X6b from the PCB

Measurement:

Test 1:
Measure the resistance between pin 4 (middle brown) and pin 5 (middle blue) wires on X6b connector.
Resistance should be approximately 160 Ohms or greater.
Test 2:
Measure the resistance between pin 4 (middle brown) on X6b connector to a suitable earth point.
Resistance should be greater than 1M Ohm.
Test 3:
Measure the resistance between pin 5 (middle blue) on X6b connector to a suitable earth point.
Resistance should be greater than 1M Ohm.