Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Boiler ON/OFF Switch OFF
CH Temperature Selector OFF
DHW Temperature Selector to Max
Ensure DHW is cool
CH Temperature Selector OFF
DHW Temperature Selector to Max
Ensure DHW is cool
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Boiler ON/OFF Switch ON.
Allow Fan to run and Burner to ignite for at least 4 seconds.
Measure Voltage across the connections of the Modulating Solenoid (blue wires).
Allow Fan to run and Burner to ignite for at least 4 seconds.
Measure Voltage across the connections of the Modulating Solenoid (blue wires).
Isolate as per TOPs P4
and refit as necessary