Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove black plug from main gas valve solenoid

Measurement:

Measure resistance across the exposed connections of the solenoid operator
(test across the two smaller pins that are opposite each other).
Resistance should be approximately 5.2kOhms.
Reconnect plug after test.