Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the air tube from the air box's air intake

Observation:

Thoroughly inspect the air box attached to the fan assembly, ensure the air box is clear of any blockages.
Note:
The fan assembly and air box do not need to be removed to carry out this inspection, it can be carried out in situ, image is for illustration purposes only.