Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove fan and the fan venturi (located in fan outlet)

Measurement:

Check fan impeller moves freely and fan blades are clean.
Inspect the venturis for restrictions or damage.
Note:
It is not sufficient to just blow through the venturis,
ensure it is removed and cleaned thoroughly with
an abrasive cloth and/or washed in water.