Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove ignition PCB from the gas valve
Disconnect leads to modulating solenoid

Measurement:

Test 1:
Measure the resistance across pin 1 and 5 of the gas valve main solenoid.
Resistance should be approximately 4.2kOhms.
Test 2:
Measure the resistance across pin 1 and 3 of the gas valve main solenoid.
Resistance should be greater than 1M Ohm.
Test 3:
Measure the resistance across the two pins of gas valve modulating solenoid.
Resistance should be approximately 115 Ohms.