Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Boiler primary circuit drained as necessary

Observation:

Visually inspect the condition of the pump impeller it should be in good condition and not restricted.
Check the pump impeller is free to rotate.
Is the pump impeller free to turn and in good condition / unrestricted?
Note:
If pump is satisfactory do not refill appliance until a diagnosis is reached.