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 of the blue wire on
the gas valve plug to pin 9 (blue) at X20.
Check for continuity of the red wire on the
gas valve plug to pin 18 (red) at X20.
Inspect harness and connectors for
good overall condition.