Set Appliance State:

Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the DC voltage between the grey wire on
the fan DC connector and a suitable earth point.
Expect a reading of approximately 16 - 22 vdc.