If I have the freezer on the highest setting(10 to 15 deg F...based on thermometer), it seems to run fine and defrosts periodically to limit frost. If I run on the next highest setting, it runs continuously even though the temperature is -12 deg F...based on thermometer. Because it is running continuously, I dont think the defroster ever comes on and it frosts up. I could keep it on the higher setting but I prefer the colder temperature. Thanks in advance, Dave