Isolate Mains Power to appliance as per TOPs P4
Remove the connector from the gas valve and orientate as shown
Remove connector CON12 from the PCB
Set multimeter to Diode test mode
(click here for how to do this)

Observation:

Expect a reading of approximately 0.6vdc on each test.
Test 1: Red probe on brown of CON12 and the black probe on the left hand pin of the gas valve connector.
Test 2: Red probe on blue of CON12 and the black probe on the left hand pin of the gas valve connector.
Test 3: Black probe on brown of CON12 and the red probe on the right hand pin of the gas valve connector.
Test 4: Black probe on blue of CON 12 and the black probe on the right hand pin of the gas valve connector.
Note:
Left / right pins refer to connector orientated as shown in the picture.