I have a 1994 Kenmore upright auto defrost freezer which is chilling to 30 below zero even on the lowest setting. When I turn it to a lower number, it shuts off for a while, but when it restarts, it chills right back to 30 below. I have checked the evaporator coil and it is not frozen up. I have cleaned the drain, which was frozen. The freezer does not have an externa condenser. I think it is the thermostat, but I was told that this is a non-returnable part, and would like another opinion, before making an unnecessary purchase. Thank you for your help.