Move the refrigerator away from walls to allow free access to the back of the appliance. Unplug it and remove access the panel around the fan with a screwdriver. Inspect fan to make sure it is free of any dust, debris or anything that would keep it from operating properly. Clean dust off of fan with dry cloth.
Turn fan a few times to make sure it is not catching on anything and is able to turn freely. Remove the fan motor with a screwdriver and pull out of motor housing.
Locate the electrical wires on the back of the motor and mark wire placements so you can remember where they go when it's time to put them back. Remove the wire harness from the motor by pulling back on the terminal plug. Make sure you do not pull on the wires themselves. If the plug is resistant, gently pull it off with a pair of needlenose pliers.
Inspect the metal connectors inside the terminal plug to verify that they are not corroded. If they are, make sure you replace the plug. Use a multitester to check the continuity of the fan motor by setting the Ohms on "X10" and connecting the probes to the terminals inside the motor.
Verify that the motor terminals are giving a measurable reading above 0 ohms. If not, the motor needs to be replaced.