Set Appliance State:

Mains Power Off
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Set Multimeter to DC Volts Set meter to measure DC Volts
Leave connector in place
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Once the fan begins to run measure the voltage between the black wire (pin 3) on connector marked X20 and a suitable earth.
Approximately 6V DC should be present.