Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the gas valve harness from the boiler and orientate as shown in the picture, using the screw to hold connector X14 in place may help to carry out the test
Set multimeter to Diode test mode
(click here for how to do this)

Measurement:

Expect a reading of approximately 0.55V DC on each test.
Test 1: Red probe on brown wire of connector X14 and the black probe on the bottom pin of the gas valve connector.
Test 2: Red probe on blue wire of connector X14 and the black probe on the bottom pin of the gas valve connector.
Test 3: Black probe on brown wire of connector X14 and the red probe on the top pin of the gas valve connector.
Test 4: Black probe on blue wire of connector X14 and the red probe on the top pin of the gas valve connector.