Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the 4 way connector (X15) from the PCB
Also disconnect the lead from the gas valve

Measurement:

Check the harness leads for continuity.
Measure the continuity of the black and green wires between either end of the harness.
Check the general lead condition.