Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

When the fan starts measure the DC voltage between pin 3 (Black) on connector X20 and a suitable earth.
Expected voltage approximately 5.6 Vdc.
Note:
Each pin on the PCB connector is numbered.