Remove all freezer drawers.

Remove all glass shelves.

Unscrew the screws on air duct.
a. Check the freezer air duct to find the three screw covers.

b. Remove the screw covers (total of three) on the air duct with slotted screw driver.
c. Unscrew the screws (total three) with Cross-head screw driver.

Dismantle the air duct.
a. Catch the bottom of air duct and pull down the air duct.
b. Pay attention to the connectors when pulling out the air duct.

c. Disconnect the terminal for fan motor.

Disconnect the terminal for freezer temp. sensor. 

d. Move the air duct out of the freezer compartment.

Check fan blade is broken or not.

Shake fan blade to check if fan is loose.

Supply DC 12V power to fan to check if there any clash.

Make sure the sealing sponges are in good condition.

When reinstalling the air duct, fasten the wires to avoid crushing with air duct.

Install the top side first when installing the air duct.

After pushing air duct into place, you should hear a clicking sound. If not, repeat again.

Check to see if there is
a wide gap between air duct and cabinet. If there is, reinstall air duct.