Set Appliance State:
Mains Power Off
Gas On; Pilot On
Set Multimeter to DC Volts 
CH Demand
Selector Switch Winter position
Gas Control Knob in Main Burner position
Gas Control Knob in Main Burner position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With the main burner alight, measure voltage across the Modulation Coil
(Grey wires) whilst varying the Boiler Temperature Potentiometer.
The voltage should vary over the range 0-13 Volts DC.
(Grey wires) whilst varying the Boiler Temperature Potentiometer.
The voltage should vary over the range 0-13 Volts DC.
Isolate as per TOPs P4
and refit as necessary