Set Appliance State:

Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch in OFF position
CH temperature selector to minimum
DHW temperature selector to maximum
Set flow rate at tap to approximately 5 l/min
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With hot water tap on, turn selector switch to the water heating position.
Allow the burner to ignite and, after approximately 3 to 4 seconds, go to maximum burner pressure (approximately 8.1mb).
Measure voltage across the terminals of the modulating solenoid, while slowly turning down the DHW temperature selector.
Does the voltage reduce as the temperature demand is met?