Frost may build on the evaporator coils on the back of the freezer. Once this happens, air can't circulate through the coils properly. Thus, the freezer will not remain cold because of a lack of cold air. To check whether there's a problem with the evaporator coils, remove perishable items from your refrigerator and freezer and unplug the unit. Open the doors and wait about 48 hours. This will allow the refrigerator to defrost. If the freezer works properly after you plug it back in, the problem is in the self-defrosting system; call a technician.
The condenser coils on the bottom of the refrigerator have to be cleaned regularly. If dirt, dust or debris build up on these coils, air won't circulate properly throughout the refrigerator. Clean your coils once a week or if you notice a problem. Unplug the refrigerator before cleaning. The coils are located on the bottom of the refrigerator; use a lint brush and your vacuum's nozzle attachment to remove dust and debris from this part of the refrigerator. These coils are different from the evaporator coils, which are located on the freezer.
Most freezers have a timer that switches the freezer from cooling mode to defrost mode and back again. This allows the freezer to automatically defrost so that frost doesn't clog the evaporator coils. If the timer fails for any reason, the freezer won't defrost properly. The timer could fail to switch into defrost mode, causing frost buildup. It can also fail to switch from defrost mode back into cooling mode, causing cool air to stop circulating through the refrigerator.
Most refrigerators have a fan in the freezer section. This fan runs whenever the compressor is running and circulates air through the freezer and refrigerator. If the fan motor breaks or the fan otherwise stops working, air won't circulate properly through the freezer and refrigerator, and the temperature inside the unit will drop. Failure of the evaporator fan can also lead to frost buildup on the evaporator coils.