Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect gas valve harness at the gas valve and at X5 on the PCB (looping cable as shown may make it easier to carry out test)
Set multimeter to Diode test mode
(click here for how to do this)

Measurement:

Expect a reading of approximately 0.5 to 0.6V DC on each test.
Test 1: Red probe on brown wire and the black probe on the top pin of the gas valve connector.
Test 2: Red probe on blue wire and the black probe on the top pin of the gas valve connector.
Test 3: Black probe on brown wire and the red probe on the bottom pin of the gas valve connector.
Test 4: Black probe on blue wire and the black probe on the bottom pin of the gas valve connector.