Set Appliance State:
Gas Off
Set Multimeter to mVolts DC 
Isolate Mains Power to appliance as per TOPs P4
Selector Switch in HW position.
Connect crocodile clip to outer sheath of Thermocouple.
Selector Switch in HW position.
Connect crocodile clip to outer sheath of Thermocouple.
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With Pilot lit depress the Gas Valve Control Knob.
Measure the voltage across the Thermocouple and the Thermostat terminal connected to the White wire which runs to the rear of the Main Gas Valve.
Measure the voltage across the Thermocouple and the Thermostat terminal connected to the White wire which runs to the rear of the Main Gas Valve.
Isolate as per TOPs P4
and refit as necessary