Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to DC Volts 
CH switch OFF
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Gas On
Mains Power On
Measurement:
Pre-heat switch ON.
Allow fan to run and main burner to ignite.
Measure voltage across the connections on the modulating solenoid (grey wires).
Allow fan to run and main burner to ignite.
Measure voltage across the connections on the modulating solenoid (grey wires).
Isolate as per TOPs P4
and refit as necessary