Sounds like a cold control. It is located behind the panel and can be ohmed out to check for proper operation. You need to remove the light shield then after the shelves have been taken out remove the two screws that hold the fan shroud in place. This will allow back panel to come for so you can check the cold control. It should read as a short in all positions except off when the cap tubes have come to ambient temp. If you get the back off and see ice on only a small portion of the evaporator you have a sealed system problem and need a qualified technician.