Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
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

Observation:

Check voltage at PCB connector X20 pin 4 (Yellow) to earth.
This is the signal from the main PCB to the Fan controlling fan speed.
The table below shown the approximate voltages during normal operation:
Voltage
At Rest7.6v
Demand10.5v