Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Fit a link between the two connections marked C and D at the front of the PCB
Main switch in ON position
Summer/Winter Switch in Winter position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

With the external controls / integral timer (if fitted) linked out does the fan speed now increase?
Note:
Remove link and refit wires after test.