Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connecter ST9 from the PCB
If available use the Worcester test PCB to assist resistance test

Measurement:

Measure resistance of pin 8 (Green wire) on disconnected connector ST9 to a suitable earth point.
Resistance should be 1M Ohm or greater.