Why is frost building up in my refrigerator's freezer?

Replace the refrigerator defrost sensor

The refrigerator defrost sensor-also known as the defrost bi-metal termination thermostat-trips when it detects that the temperature of the evaporator is getting hot enough that it might overheat. When the defrost sensor trips, it shut off power to the defrost heater. If the defrost sensor trips because it's not working correctly, frost builds up on the evaporator fins, eventually making the refrigerator and freezer not cool well. You can use volt/ohm meter to check the defrost sensor for continuity. Replace the defrost sensor if it shows no continuity at around 0 degrees F.

Repair the refrigerator defrost system

The refrigerator automatic defrost system periodically melts frost from the evaporator to improve heat exchange. During defrosting, the compressor stops, the defrost heating element turns on, and frost melts from the evaporator fins. The condensate drips to a evaporator drip tray below the evaporator and then flows through a defrost drain tube to a drain pan next to the compressor in the machine compartment. The condensate water evaporates from the drain pan before the next defrost cycled. If the defrost process fails, diagnose and repair the problem.

Replace the refrigerator electronic control board

The electronic control board-also called the main control board or the power control board (PCB)-controls the compressor and the defrost cycle. When it senses that the compressor must run to keep the refrigerator cool, it sends voltage to the compressor and fans. The electronic control board also receives signals from temperature sensors to monitor the temperatures inside the refrigerator and freezer. With this information, the electronic control board controls the defrost cycle. You can usually do a diagnostic test on the electronic control board to see how well it's working. The test varies by model, but the most common is the Forced Defrost test. See the tech sheet for instructions on running the diagnostics. Replace the electronic control board if it's not working correctly.

Adjust the freezer or refrigerator door

The refrigerator or freezer doors swing on hinges that can be damaged or bent. Over time, the doors may begin to sag, allowing warm moist air into the refrigerator or freezer door that creates excessive frost that can eventually cause the drain tube to freeze. This refrigerator repair involves adjusting or repairing the hinges so that the doors work properly and stay aligned.

Replace the refrigerator defrost timer

The defrost timer is an electro-mechanical device that controls the intervals between automatic defrost cycles in the refrigerator. The defrost timer motor runs and moves the control components in the device. When the control contacts in the defrost timer advance into the defrost cycle, the compressor stops and the defrost heater turns on for a specified period of time to melt frost off of the evaporator fins. This promotes more efficient exchange of heat across the evaporator. When the specified period of defrost ends, the defrost timer contacts switch back to allow normal cooling operation in the refrigerator.

You'll need to replace the defrost timer if it doesn't advance when activated.

Faulty contacts in the timer can also cause the defrost heater to either not energize at all or constantly energize. In that instance, replace the defrost timer.

Replace the refrigerator door gasket

The door gasket attaches to the refrigerator or freezer door panel and prevents air from entering or escaping the cabinet when the door is shut. A damaged gasket lets warm, moist air into the refrigerator when the door is closed, causing excessive frost and cooling problems. Replace the door seal if it's damaged or torn.