Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect plug from the gas valve

Measurement:

Check for continuity (zero resistance) between the ends of the red and black wires running from the gas valve to pins 1 and 2 at X4 on the PCB.
Inspect the harness and connectors for good overall condition.