Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the connector plugs from the gas valve (violet and blue leads) and unplug the large (12 way) connector for the right hand side of the PCB

Measurement:

Check harness for continuity across the violet gas valve leads and blue leads at positions 6, 7, 8, & 9 at the PCB end to the gas valve connectors.
Check the general lead condition.