Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect lead 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:
Test between each of the four pins (1, 2, 4 and 5)
to a suitable earth point.
Resistance should be greater than 1MOhm on each test.