Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Mode selector switch to DHW position.
Turn DHW stat to maximum.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to light and check for approximately 9 Vdc on the purple and grey wires at the modulation coil.