Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4

Measurement:

Set thermostat to OFF position.
Measure the resistance across connection 24 (yellow lead) and connection 25 (yellow lead) on the control module. Resistance should be open circuit.
Set thermostat to MIN position.
Measure the resistance across connection 24 (yellow lead) and connection 25 (yellow lead) on the control module. Resistance should be closed circuit.