While it may seem strange, having only the top half of your freezer not freezing is a common refrigerator problem. If you have a Frigidaire freezer that is experiencing this problem, consult your instruction manual to find out where the fan system is in your freezer. You may need to pull the freezer away from the wall and access the back paneling in order to find the source of the problem.
When the top half of a freezer is not working properly it is a sign that the air circulation within the freezer is not functioning. In other words, the freezer refrigerant is still working properly, but the cold air it produces isn't going anywhere: Instead, it is sinking to the bottom of the freezer (heat rises) where it does an excellent job of keeping that half of the freezer cold while missing the top section entirely.
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The air circulation problem is usually caused by a fan issue. Find where your fan connects to the refrigerant system and pumps the air out into the freezer. If the fan blades are stuck or frozen, you may be able to loosen them or replace the fan. If the fan works mechanically but does not turn on the problem lies with the motor instead. In this case, you can replace the motor or call in a professional for more detailed electrical tests.
If your fan is working properly, the issue could be with the filter that your freezer uses. These filters, just like the AC filters in your house, can become clogged with contaminants over time. This might slow down the airflow so much that it cannot properly circulate through the freezer. Order a new filter from the Frigidaire manufactures and replace it.
The evaporator is the section of the refrigerator that helps the refrigerant to shed its heat and resume a gas-like shape. This evaporator uses coils that can ice over or become covered with dust, impeding performance. If the evaporator is not working correctly, it can create a chain of problems with efficiency that will reduce the effectiveness of circulation. Call in a professional to diagnose the problem or clean off the coils yourself to see if this solves the problem.