Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect connector M15 from the PCB

Measurement:

Test 1:
Measure the resistance across the brown and blue wires of M15 connector.
Resistance should be approximately 155 Ohms or greater.
Test 2:
Measure the resistance between the brown wire on M15 connector and a suitable earth point.
Resistance should be at least 1M Ohm.