If you have a frost-free freezer, it shouldn't be icing up. What can you do if it is? Freezers should self-defrost three times a day, due to a self-defrosting system built in to the freezer. If something is wrong with the self-defrosting system, the freezer will still continue to try to cool itself off. Eventually, the evaporator coolers will become covered in frost and will not work properly. If the self-defrosting system is not working correctly, there are three components that might be at fault: the defrost timer, the defrost heater and the defrost thermostat.
First, check the defrost timer. To do this, manually move the defrost timer to the defrost setting. If the freezer does not exit the defrost setting in 45 minutes, the timer is faulty and must be replaced.
Unplug the freezer.
To check the defrost heater, remove the rear panel from the back of the freezer with a screwdriver. (You may have to remove the food and the freezer's shelves to reach the panel.)
Pull the two connectors off the terminals and inspect them for corrosion. If they are corroded, replace them.
Set the multitester to ohms setting X1. Place each of the two probes on the multitester on one of the terminals, and make sure that the multitester reads somewhere between 0 and infinity. If it does not, the heater will need to be replaced.
If both the timer and the heater are operating correctly, the thermostat is probably defective. Replace the thermostat to stop the freezer from icing up.
To make sure that the defrost system is at fault, manually defrost the freezer and then turn it on again. If the freezer then works correctly, the defrost system is definitely the culprit.
Do not check the defrost heater if the freezer is running.