Set Appliance State:

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

Measurement:

Measure for resistance across pins 3 & 4 (purple)
and 5 & 6 (purple) of ST9 connector.
Expected resistances should be as table below.