Set Appliance State:

Mains Power Off
Gas On; Pilot On
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Selector Switch ON
Gas Control Knob in Main Burner position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across Modulation Coil (Grey wires).
Voltage should be approximately 6-13 Volts DC.