Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the burner to ignite and, after approximately 3 to 4 seconds, measure the voltage across the connections of the modulating solenoid.
Expected voltage is approximately 16 Volts DC.