Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector from gas valve

Measurement:

Check for continuity (zero resistance) of the black and blue wires running from the disconnected plug to X2A on the PCB.
Blue = X2A Pin 1
Black = X2A Pin 2.
Inspect harness and connectors for good overall condition.
Note:
Wires may be multicoloured.