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
Pull X3 back slightly to expose test pins

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.