Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Primary system water drained and discharged
Remove the pump head

Observation:

Inspect the filter located in the rear of the pump housing.
It should be clear and unrestricted.
Is the filter clear and unrestricted?
Note:
It should be possible to ascertain the condition of the filter as described above however if a more detailed inspection is required the auto air vent can be removed and the filter withdrawn upwards.