Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove black plug from the upper solenoid on the gas valve
Remove black plug from the upper solenoid on the gas valve
Measurement:
Measure the resistance across the two upper terminals on the gas valve solenoid.
Resistance should be approximately 3.8 kohms.
Resistance should be approximately 3.8 kohms.
Isolate as per TOPs P4
and refit as necessary