Set Appliance State:

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

Measurement:

Test 1:
Measure resistance across terminals 1 and 4 of the gas valve.
Expected resistance approximately 4200 Ohms.
Test 2:
Measure resistance from terminal 1 to earth.
Expected resistance open circuit.
Test 3:
Measure resistance from terminal 2 to earth.
Expected resistance open circuit.