If your refrigerator isn't cooling and you can see frost buildup on the inside back wall of your freezer, this video from Sears PartsDirect can help you troubleshoot and fix the problem. We'll explain how the defrost system’s main components—the electronic control board, defrost heater, thermistor and bi-metal thermostat—work, and how to determine which one is responsible for the excessive frost.
Check out our DIY refrigerator repair page for more how-to guides and videos, answers to common questions and error code charts.
Hi, my name is Wayne with Sears PartsDirect. Today, we're going to talk about troubleshooting a refrigerator’s defrost system. For some of these tests, you’ll need a multimeter. If you don’t have one, you can find one at Sears.com.
You’re probably watching this video because you see this, and your refrigerator isn’t cooling very well. Your refrigerator is supposed to automatically defrost that evaporator every 8 hours or so. When something goes wrong with the defrost system, frost builds up and clogs the passages that move cool air from the freezer to the refrigerator, causing it to warm up. So what went wrong? That’s what we’ll help you find out.
Defrost system problems
The defrost system has four main parts. The electronic control board starts the defrost cycle and regulates how often they are triggered. If the control fails, your refrigerator can’t defrost automatically. The thermistor, also called the defrost temperature sensor, senses evaporator temperature and sends signals to the electronic control board.
If the thermistor senses the wrong temperature, the electronic control board won’t turn on the defrost heater. The defrost heater melts the frost off the evaporator fins. If the heater breaks, the frost stays put. If the defrost bi-metal thermostat is defective, then the heater won’t turn on. So where do we start?
Defrost heater checks
First, let's see if the defrost heater turns on using the forced defrost mode. On this Kenmore model, press the door switch 5 times within 2 seconds to enter the forced defrost mode. The control will beep and then the defrost mode starts. If you ever need to exit this mode, just unplug the refrigerator. If this doesn’t work on your model, check your tech sheet for instructions. You can usually find it located behind the bottom front grill or taped to the back of the refrigerator.
Check to see if the heater heats up. If your heater heats up, the frost will start to melt. If the defrost system heated up in the test mode, click here to skip to the next section and learn how to check resistance on the thermistor. If the defrost heater doesn't work in the forced defrost mode, you’ll need to check the continuity through the defrost heater circuit from the electronic control board.
To check the defrost heater circuit, the refrigerator temperature needs to be below 46 degrees. If it’s above 46 degrees, plug your refrigerator back in to bring the temperature back down. For safety’s sake, always make sure the refrigerator is unplugged before you check continuity. Drop the control panel down to access the electronic control board. Put one meter lead on the brown wire and the other meter lead on the light blue wire. If the meter reads about 40 ohms then you have continuity.
Bad thermistor or electronic control board
If your defrost heater circuit shows continuity, your problem is with the electronic control board or thermistor because the electronic control board isn't sending current through the defrost heater circuit. Let’s check the thermistor resistance next. With the refrigerator unplugged, remove the back freezer panel.
Put one meter lead on each of the gray/white wires in the harness plug. You should measure around 63,000 ohms at 0 degrees. If the freezer is not at 0 degrees, compare your resistance reading to the temperature resistance chart found on your tech sheet. If the thermistor resistance reading is off by more than 10 percent, the sensor (thermistor) needs to be replaced. Here’s a video that will show you how.
If your thermistor checks okay, you'll probably need to replace the electronic control board. This video will show you how to do that.
Faulty heater or bi-metal thermostat
If you didn't measure continuity through the heater circuit when it cooled down, then you could have a broken heater or a defective bi-metal thermostat.
Check the defrost heater and replace it if it’s visibly damaged. Even if your defrost heater doesn't appear to be damaged it can still be broken. Check the continuity using a multimeter. Unplug the heater from the wire harness. Put the red meter lead one of the heater wires and the black meter lead on the other. You should measure about 40 ohms of resistance through the defrost heater. If the meter displays 1 in the far left digit indicating the defrost heater is electrically open, it will need to be replaced. This video will show you how.
If the defrost heater is okay, check the bi-metal thermostat for continuity. With the refrigerator unplugged, put one meter lead on the light blue wire in the harness plug and the other lead on the blue wire at the defrost bi-metal thermostat. If the meter reads near zero ohms then you have continuity. Replace the defrost bi-metal thermostat if it's electrically open. This video will show you how to do that.
Well, I hope this video helped you out today. Be sure to check out our other videos here on the YouTube channel, and don’t forget to subscribe.
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