Please help, I have GE side by side refrigerator. The freezer side is cold(below set point) but the refrigerator side is warm. I removed the panel from inside the back of the freezer and the coils are all iced up. How can I make sure which part is the one that needs replaced. Heater, thermostat? What other things could it be. I don't have any extra money so I need to make sure I am replacing the correct part. Thanks in advance.