Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X25 from the PCB
Disconnect the stepper motor lead from the gas valve

Measurement:

Measure the continuity of each of the leads in the harness from one end to the other.
Red to Red
Green to Green
White to White
Black to Black
Expect continuity or zero resistance on each wire in the harness.