Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Boiler Switch in the OFF position
Remove the connector on pins 3, 4 & 5 on PCB

Measurement:

Measure short circuit resistance across Red and Purple wires of disconnected connector.
Measure open circuit resistance across Red and Grey wires of disconnected connector.
Replace connector after test.
Note:
Measurement taken at connector NOT at PCB.