Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector plug from gas valve

Measurement:

Test 1:
Measure the resistance between pins 1 and 2 on the gas valve.
Resistance should be approximately 1.6kOhms.
Test 2:
Measure the resistance between pins 4 and 5 on the gas valve.
Resistance should be approximately 2.8kOhms.
Test 3:
Measure the resistance from pin 1 to a suitable earth point.
Measure the resistance from pin 5 to a suitable earth point.
Resistance should be above 1M Ohm (or O.L.) on both.