Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the plug from the gas valve
Disconnect connector X20 from the PCB

Measurement:

Check for continuity (zero resistance) of the two wires at the gas valve plug to pins 9 and 18 at X20 on the PCB.
Inspect harness and connectors for good condition throughout.