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 blue and black wires running from the disconnected plug to X3 on the PCB.
Black = X3 Pin 1
Blue = X3 Pin 3.
Inspect harness and connectors for good overall condition.
Note:
Wires may be multicoloured e.g. blue & white.