Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the 4 way connector from the PCB
Remove the electrical connector from the gas valve

Measurement:

Test 1.
Measure the resistance between pin 3 (brown) on the
PCB connector to pin 2 on the gas valve connector.
Test 2.
Measure the resistance between pin 3 (brown) on the
PCB connector to pin 4 on the gas valve connector.
Test 3.
Measure the resistance between pin 4 (blue) on the
PCB connector to pin 1 on the gas valve connector.
Test 4.
Measure the resistance between pin 4 (blue) on the
PCB connector to pin 5 on the gas valve connector.
Expect continuity / zero resistance on each test.
Also visually inspect the condition of the harness.