Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the connector from the gas valve and orientate as shown when testing
Disconnect X1 from the PCB
Set multimeter to Diode test mode
(click here for how to do this)

Observation:

Expect a reading of approximately 0.60 vdc on each test.
Test 1: Red probe on pin 2 of connector X1 and the
Black probe on pin 5 of the gas valve connector.
Test 2: Red probe on pin 3 of connector X1 and the
Black probe on pin 5 of the gas valve connector.
Test 3: Black probe on pin 2 of connector X1 and the
Red probe on pin 1 of the gas valve connector.
Test 4: Black probe on pin 3 of connector X1 and the
Red probe on pin 1 of the gas valve connector.
Note:
It can be difficult to make contact with the pins inside the gas valve connector, take care to ensure a good contact is made.