What Causes Frost on the Evaporator of a Refrigerator?

Timer Failure

• Frost-free refrigerators use timers to switch back and forth from cooling mode to defrost mode. If the timer fails while in cooling mode, the refrigerator won't switch to defrost mode. Frost thus builds up on the evaporator coils and doesn't defrost unless you manually defrost your refrigerator. If this problem isn't taken care of, the frost will clog the evaporator coils and air won't flow through the refrigerator.

Defrost Heater Problem

• When the timer switches to defrosting from cooling, the defrost heater is supposed to turn on to melt any frost built up on the evaporator coils. However, sometimes the defrost heater burns out or otherwise stops working. If this happens, the heater won't turn on, so the frost won't melt, even if the refrigerator switches back and forth from cooling to defrost modes properly.

Evaporator Fan Motor Dies

• Most freezers employ an evaporator fan to keep air circulating through the freezer and refrigerator. This fan runs whenever the compressor is running so that the air in the freezer stays at a constant temperature. The fan then turns off during the defrost cycle so the evaporator coils can defrost. If the fan motor stops working, however, air won't circulate during the cooling cycle and frost will accumulate on the evaporator coils. Since the air isn't circulating properly, the freezer and refrigerator temperature will drop.

Thermostat Problem

• The thermostat triggers the defrost timer to move from one cycle to another. This part signals the defrost timer to move into defrost mode when the freezer reaches a pre-set temperature. When the temperature inside the freezer gets too warm, the thermostat signals the defrost timer to move back into cooling mode. Thus, if the thermostat stops working, the timer will get stuck in one or the other cycles, and if it happens to get stuck in the cooling cycle, frost will accumulate on the evaporator coil.