Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Selector switch to DHW position
CH temperature selector to Min
DHW temperature selector to Max
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage at the modulating solenoid and a suitable neutral.
Approximately 15-20 volts should be present.