Set Appliance State:

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

Measurement:

Test 1.
Measure the resistance between pin 8 (grey) on
connector X4 and pin 3 on the gas valve connector.
Test 2.
Measure the resistance between pin 17 (grey) on
connector X4 and pin 1 on the gas valve connector.
Expect continuity / zero resistance on each test.
Also visually inspect the condition of the harness and connectors.