A Faulty Thermistor Can Cause A Refrigerator To Run Constantly

Does your refrigerator seem to be running all the time? Or is the refrigerator compartment having trouble staying cool due to the evaporator coils not defrosting properly? These problems could be caused by one of the smallest components you can find in the appliance: a thermistor. Repair Clinic has an easy way to test a refrigerator thermistor to help determine if it is, indeed, the cause of your refrigerator’s problems, but let’s first take a look at what a thermistor actually does.

What is a thermistor?

The thermistor is a small semiconductor and resistor, about the size of a Lima bean, which reacts to temperature change. This sensor may monitor the air temperature in the refrigerator or the freezer compartment, or it may monitor the temperature of the evaporator. Can one thermistor do all this? No, which is why refrigerators may actually have more than one thermistor installed in the appliance (some may have as many as four).

Where can you find a refrigerator thermistor?

Refrigerator and freezer compartment thermistors may be attached to the interior liner of either compartment, usually behind a cover, or the thermistor may be located behind a rear compartment panel or an air duct. If the thermistor is being used to monitor the temperature of the evaporator, the component will usually be attached directly to an evaporator refrigerant tube. The thermistor responds to temperature changes within the compartments or on the evaporator by sending a signal to the refrigerator’s control board.

The thermistor signals the control board when to allow voltage to travel to the compressor and fan motors

When the thermistor detects that the air temperature in the appliance has risen above a certain level, it signals the refrigerator control board to allow voltage to travel to the compressor as well as to the condenser fan motor and the evaporator fan motor. When cycled on, the compressor pumps refrigerant in gas form into the condenser coils where the gas is then condensed into a hot liquid. The condenser coils dissipate the heat as the liquid travels through them which is assisted by the condenser fan motor blowing air across the coils. The refrigerant is then pumped into the evaporator coils where it expands back into a gas. This makes the coils cold. The evaporator fan motor draws air over the cold evaporator coils which chill the air before that air is circulated through the freezer and refrigerator compartments.

Once the air temperature in the compartments is cold enough, the thermistor alerts the control board to shut off the voltage to the cooling system. Keep in mind that it’s normal for the temperature to vary throughout this cycle.

The control board depends on the defrost thermistor to determine if the evaporator is cold enough for defrosting

About twice a day, every 8 to 10 hours, the control board will turn on the defrost heater to defrost the evaporator. The control board depends on the defrost thermistor to determine if the evaporator is cold enough to turn the heater on, usually around 15° F (-9° C), and when to turn the heater off, usually around 50° or 60° F (10° or 16° C). If it takes a long time to melt the frost off the evaporator coils, the control board may shorten the time between defrost cycles; if the defrosting occurs relatively quickly, the control board may lengthen the time between defrosting cycles. This is known as an adaptive defrost cycle and is necessary to allow good air flow to pass through the evaporator to maintain the appropriate temperature in the refrigerator.

What is the appropriate temperature for your refrigerator?

What’s an appropriate temperature for your refrigerator to maintain? Repair Clinic recommends the refrigerator compartment temperature be kept between 35° F and 40° F (2° C to 4° C) for the appliance to work optimally; the freezer compartment temperature should be kept at 0° F ( -18° C) or lower.

How a faulty thermistor can affect your refrigerator

A faulty thermistor may not be able to detect the change in temperature within the refrigerator or freezer compartments, which can result in the appliance not cooling sufficiently or in the compressor running all the time (since the control never receives a signal that the refrigerator or freezer compartment has reached an appropriate temperature). If a defrost thermistor is unable to detect that the evaporator coils are cold, the defrost heater may not turn on and the coils will frost over.

An easy way to test a refrigerator thermistor

There’s an easy test you can perform with a multimeter and a hair dryer to determine if a thermistor can respond correctly to temperature change, but first, you will need to isolate or remove the component from the appliance. This will require detaching a liner cover or removing a rear compartment panel or air duct (be sure to unplug the refrigerator’s power cord before attempting any disassembly). To fully uninstall some thermistors, you will just have to disconnect a wire connector; other thermistors will require you to use wire cutters to sever the wires four or five inches below the thermistor to remove the component.

Inspect the thermistor for an ohm rating

Once the thermistor has been removed from the refrigerator, inspect the component for a designated ohm rating printed on the sensor casing or wire, then rotate the multimeter’s range selection dial to the corresponding setting for ohms of resistance. No designated ohm rating on the thermistor? Check the refrigerator’s tech sheet (often found behind the appliance’s lower grille) or just start by rotating the multimeter’s dial to the lowest setting for ohms of resistance then go to higher settings as needed until you get a reading.

Testing the thermistor for resistance at room temperature

Contact the black meter lead to a thermistor wire or connector terminal and the red lead to the other wire or terminal. At room temperature, the multimeter display should indicate close to the ohms of resistance designated on the thermistor itself or in the refrigerator’s tech sheet. Readings can vary depending on the thermistor and its manufacturer, but an average will fall between 7,000 ohms to 40,000 ohms of resistance. More important is if the meter display indicates no ohms of resistance; if this is the case, you’ll know right away that the thermistor is faulty and a new one will need to be installed.

Test the thermistor for changes in resistance as the temperature changes

You should also confirm the thermistor’s ohms of resistance will change as the temperature changes. This is where the hair dryer comes into play. With the meter leads still attached to the thermistor’s terminals or wires, set the hair dryer to a heat setting, turn it on, and aim the nozzle directly at the thermistor. As the thermistor heats up, the meter display should show the ohms of resistance decreasing or increasing (either response is acceptable as thermistors are made to react differently to changes in temperature). If the resistance changes, the thermistor should be functional. However, if you observe no changes in ohms of resistance, the thermistor is likely defective and should be replaced.

Refrigerator Thermistor Testing

Refrigerator repair know-how from Repair Clinic

While testing a thermistor is relatively easy, locating and uninstalling the component may be more difficult. That’s why Repair Clinic provides you with the refrigerator repair know-how you need to get the job done. Enter your refrigerator’s full model number in the Repair Clinic website’s “Videos & Articles” search bar to find model-specific schematics, step-by-step guides, and procedural videos such as this one showing how to reach a thermistor attached to the evaporator on a GE Side-By-Side Refrigerator (Model GSS25JEPECC) or this video detailing how to reach a thermistor installed behind an air duct in the refrigerator compartment on a Frigidaire French Door Gallery Refrigerator (Model FGHB2866PFGA) as well as a freezer compartment thermistor on the same model.

Find specific refrigerator parts at Repair Clinic

There are numerous refrigerator thermistors out there specifically designed to work on models manufactured by Samsung, Frigidaire, KitchenAid, GE, Kenmore, LG, Maytag, Whirlpool, and others. To help you find the right one, just enter the refrigerator’s full model number in the Repair Clinic website search bar, then choose “Sensor & Thermistor” using the “Part Category” navigation filter followed by the appropriate entry listed in the “Part Title” filter. Whatever thermistor or sensor you need, you can be sure you’re getting a genuine manufacturer part from Repair Clinic, your repair partner.

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