Set Appliance State:
Mains Power Off
Set Multimeter to mVolts DC 
Turn Hot Water Tap On
Gas On
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Depress the gas control knob.
Allow the boiler to execute its purge period and the pilot to ignite.
Keep the gas control knob depressed, and measure the millivoltage between the thermocouple interrupter and a suitable earth point.
Allow the boiler to execute its purge period and the pilot to ignite.
Keep the gas control knob depressed, and measure the millivoltage between the thermocouple interrupter and a suitable earth point.
Isolate as per TOPs P4
and refit as necessary