Isolate Mains Power to appliance as per TOPs P4
Remove the connector from the gas valve
Position connector as shown in bottom picture
Set multimeter to Diode test mode
Click here for how to set multimeter

Measurement:

Expect a reading of approximately 0.55vdc on each test.
Test 1: Red probe on pin 26 of connector X15 and the black probe on the top pin (1) of the gas valve connector.
Test 2: Red probe on pin 27 of connector X15 and the black probe on the top pin (1) of the gas valve connector.
Test 3: Black probe on pin 26 of connector X15 and the red probe on the bottom pin (5) of the gas valve connector.
Test 4: Black probe of pin 27 of connector X15 and the red probe on the bottom pin (5) 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. Pins on connector X15 are numbered on PCB.