How to Troubleshoot Defrost System Problems in Refrigerators

This #diy troubleshooting video shows how to #diagnose and #repair a refrigerator that isn't cooling because the automatic defrost system isn't working.

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If you see frost on the inside back wall of the freezer compartment, and your refrigerator isn’t cooling well, this video helps you identify the problem.

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Check the defrost heater
First, check if the defrost heater turns on when in forced defrost mode. This test narrows down the cause of the problem to EITHER the thermistor or electronic control board OR the defrost heater, defrost bi-metal thermostat or wiring.
On this Kenmore model, press the door switch 5 times within 2 seconds to enter the forced defrost mode. The control beeps and then the defrost mode starts. (If this method doesn’t work for your model, check the refrigerator tech sheet for instructions —it’s behind the bottom front grille or taped to the back of the refrigerator).
Check whether the frost starts to melt; if so, the heater works in forced defrost mode.
If the defrost system heated up in the forced defrost mode, the heater is fine. Unplug the refrigerator to exit the test mode and skip to the section called “Test the defrost heater and bi-metal thermostat.”
If the defrost heater doesn't work in the forced defrost mode, check for the uninterrupted flow of electric current—the continuity—through the defrost heater circuit from the electronic control board by following these steps:
1. Unplug the refrigerator, for safety.
2. Drop the control panel down to access the electronic control board.
3. Check the defrost heater circuit by putting one multimeter lead on the brown wire and the other lead on the light blue wire. If the multimeter reads about 40 ohms, the heater is good.

Test the thermistor
If your defrost heater circuit shows continuity, the problem is either the thermistor or the electronic control board — one of these components is keeping the electronic control board from sending current through the defrost heater circuit. To find out which it is, test the thermistor. (You might need the tech sheet to decode the test result. Look for it behind the bottom front grille or taped to the back of the refrigerator, if you didn’t use it when checking the defrost heater.)
1. With the refrigerator still unplugged, remove the back freezer panel.
2. Take the temperature inside the freezer. You’ll need it for the next step
3. Put one multimeter lead on each of the gray/white wires in the harness plug. The multimeter should measure around 63,000 ohms when the freezer temperature is at 0 degrees. If the freezer isn’t at 0 degrees, check the chart on the tech sheet for what the resistance reading should be for the temperature of the freezer. If the thermistor resistance reading is off by more than 10 percent, replace the thermistor. Watch our video that explains how to replace a thermistor.
If your thermistor checks okay, you'll probably need to replace the electronic control board and you don’t need to continue with further troubleshooting. Watch our video that shows how to replace the control board.

Test the defrost heater and thermostat
Do this section only if you jumped to here from “Check the defrost heater.”
If you didn't measure continuity through the heater circuit, the problem is either a broken defrost heater or a defective bi-metal thermostat. Here’s how to pinpoint the problem.
First, check the defrost heater and replace it if it’s visibly damaged.
If your defrost heater doesn't show damage, follow these steps to test it:
1. Unplug the refrigerator.
2. Unplug the heater from the wire harness.
3. Put the red multimeter lead on of the heater wires and the black meter lead on the other. The multimeter should measure about 40 ohms of resistance through the defrost heater. If not, replace the defrost heater. What our video that explains how to replace a refrigerator defrost heater.
   • How to Replace a Refrigerator Defrost...  
If the defrost heater is okay, check the bi-metal thermostat for continuity. Put one multimeter lead on the light blue wire in the harness plug and the other lead on the blue wire of the defrost bi-metal thermostat.
• If it reads oL, the thermostat is broken. Watch our video that shows how to replace a refrigerator thermostat.
   • How to Replace a Refrigerator Defrost...  
• If the meter reads near 0 ohms, the thermostat has continuity and is okay.