A faulty defrost heater system usually shows up as frost buildup on the evaporator coils, weak cooling in the fresh-food section, or a refrigerator that seems colder at first and then gradually gets worse. The main parts to check are the defrost heater, defrost thermostat or termination switch, and the defrost timer or control board.
What the defrost system does
The defrost system melts frost off the evaporator coils so air can move freely through the refrigerator. When it works correctly, the fridge alternates between cooling and short defrost cycles. When it fails, ice keeps building up until airflow is blocked and temperatures rise.
Signs of failure
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Heavy frost or ice on the back panel of the freezer.
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Weak cooling in both the freezer and refrigerator compartments.
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Food staying cold but not freezing properly.
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Temperature settings no longer making much difference.
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A unit that works again after manual thawing, then fails again a few days later.
These symptoms point more toward airflow blockage from frost than a sealed-system or compressor problem. If the evaporator coil is packed with ice, the defrost system is the first place to look.
Safe troubleshooting steps
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Unplug the refrigerator before opening panels or touching wiring.
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Remove food and take off the freezer panel that covers the evaporator coils.
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Inspect the coils for frost or a solid block of ice. If the coils are heavily iced, the defrost system is not doing its job.
If the coils are frozen over, thaw them with a hair dryer on a low setting or by letting the unit sit unplugged. Do not use knives or ice picks, because they can puncture tubing and cause permanent damage.
Testing the parts
The defrost heater can be tested with a multimeter set to ohms. Disconnect the heater wires and check for continuity across the heater terminals; many heaters read somewhere around 10 to 150 ohms, though exact values vary by design. If the meter shows an open circuit or a reading far outside the expected range, the heater is likely bad.
The defrost thermostat must be cold before testing. When cooled below its closing temperature, it should show continuity; if it stays open when cold, it may be faulty. A thermostat that never closes can stop the heater from energizing during the defrost cycle.
If both the heater and thermostat test good, the defrost timer or electronic control board is the next likely suspect. That part tells the refrigerator when to enter defrost mode, so a failure there can make the system skip defrost entirely.
What each result means
When to call for help
Call an appliance technician if you see burned connectors, damaged wiring, or repeated frost buildup after manually defrosting the unit. Also get help if the refrigerator uses a sealed rear system or if removing panels exposes wiring you are not comfortable testing.
A reliable defrost repair usually starts with frost inspection, then moves to heater and thermostat tests, and finally the timer or control board. Once the defective part is identified, restoring normal airflow often brings the refrigerator cooling back to normal.