Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the air intake duct from the fan assembly

Observation:

Visually inspect the air intake / venturis on the fan assembly and the fan scroll.
They should be clear of any restrictions or blockages.
Note:
The fan does not need to be removed to carry out this inspection as it is possible with a torch and mirror.
Are the fan intake / venturis and scroll in good condition and clear of any restrictions or blockages?