Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Continuity Set meter to measure Continuity
Disconnect the 4 way connector ( X36) from the PCB
Also disconnect the lead from the gas valve

Measurement:

Check the harness leads for continuity.
Measure across the exposed ends of the disconnected harness to the other end and check all the leads individually
Check the general lead condition.