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 and unplug
connector ST9 from the right hand side of the PCB
If available use the Worcester test PCB to assist resistance test

Measurement:

Check harness for continuity, PCB connector ST9 pins 3 & 4 (purple)
and 5 & 6 (purple) to the gas valve connectors.
Visually inspect the harness condition.