Air Conditioner Not Cooling? Test The Thermistor

Home » Air Conditioner Not Cooling? Test The Thermistor

A properly-operating window or wall air conditioner can make your home a pleasant refuge from the often brutal summer heat; a malfunctioning air conditioner can leave your home feeling like a brick oven. To keep the temperature of your living space comfortable enough so you don’t feel like you’re trying to cook a pizza in it, Repair Clinic has some troubleshooting steps you can take to identify the source of your window or wall air conditioner’s malfunction as well as guide you through testing one or more of the unit’s thermistors, a potential culprit if the air conditioner is not cooling properly.

Check these 3 things first if your AC unit is not cooling properly

When troubleshooting a window or wall air conditioner that’s not cooling properly, you should check these three likely causes first:

1)  Clogged air filter.

It’s the simplest thing to check, but one that can easily prevent your air conditioner from functioning the way you need it to. A clogged or dirty air filter will greatly reduce the cooling capacity of the AC by limiting the air flow through the unit. Window and wall air conditioner filters are designed to be easily removed. You can usually just detach the unit’s inlet cover to access and remove the filter. You can clean a dirty air filter by vacuuming off any heavy soil and running water through it, something you should consider doing monthly when the air conditioner is being used frequently. Does the filter appear damaged? Then you should replace the filter with a new one.

2)  Dirty condenser coils.

Dust and dirt will build up on the inside (the fan side) of the air conditioner’s condenser coils which can prevent the AC unit from cooling properly. The coils can be cleaned by blowing compressed air through them or by using a soft bristle brush and a spray bottle of household cleaner to wipe the dirt off. You can also use a dedicated HVAC condenser coil cleaner such as Triple-D Universal Coil Cleaner. You’ll need to completely remove the air conditioner cabinet to access the coils and we recommend doing this cleaning outdoors as it can get messy.

3)  Failed fan motor.

If you can hear the air conditioning unit running, but the air doesn’t seem to be recirculating, it’s likely the fan motor has burned out. You should first confirm that the fan blades or blower wheel are unobstructed by debris. Next, you can use a multimeter to test the fan motor for electrical continuity. You should also consider testing the unit’s run capacitor to confirm it can store and release the proper electrical charge to run the fan motor (keep in mind, a faulty run capacitor will likely not be able to provide a sufficient charge to the unit’s compressor either, so you probably won’t hear the AC running at all).

Testing the air conditioner’s thermistor

If the fan motor appears to be functional, and the air filter and condenser coils are clean, but the AC still isn’t cooling properly, it’s possible one of the unit’s thermistors is out of factory specification. To help determine this, you can use a multimeter to test the thermistor for electrical continuity and for changes in ohms of resistance as the room’s temperature changes.

Unplug the air conditioner before disassembling

To conduct the test, you will need to disassemble the air conditioning unit in order to uninstall or isolate the thermistor. Before you attempt this, you should unplug the unit’s power cord to prevent electrical shock.

There may be more than one thermistor in the unit

Depending on the air conditioner model, there may be more than one thermistor in the unit; the component may be located on the front of the evaporator coils or attached to a condenser coil refrigerant tube. You will need to disconnect the thermistor’s wire connector to isolate or remove it; the wire connector may be located in a service box or attached directly to the control board.

Inspect the thermistor for an ohm rating

Once the thermistor you wish to test has been removed or isolated from the air conditioner, inspect the component for a designated ohm rating printed on the casing or wire covering. You should then rotate the multimeter’s range selection dial to the corresponding setting for ohms of resistance. No ohm rating printed on the thermistor itself? You can look for the rating on the wiring diagram or tech sheet for the AC unit or simply start at the lowest setting for ohms of resistance and rotate the meter dial to higher settings as needed to get a reading.

Contact the meter leads to the thermistor’s wire connector terminals

Contact the black meter lead to the thermistor wire or connector terminal you’re testing and the red lead to the corresponding wire or terminal. At room temperature, the meter display should indicate close to the ohms of resistance identified on the thermistor or on the air conditioner’s tech sheet. If the meter display shows no ohms of resistance, you’ll know the thermistor has failed and a new one will need to be installed.

Use a hair dryer to test for changes in ohms of resistance

You should also confirm that the ohms of resistance can change as the temperature changes. To do this, keep the black and red meter leads in contact with the wire connector terminals and use a hair dryer to heat up the thermistor. Depending on the thermistor, the meter display should show the ohms of resistance either increasing or decreasing as the thermistor gets hotter. If the resistance changes, the thermistor should be functional. However, if there is no change in resistance, the thermistor is likely defective and should be replaced.

Air Conditioner Thermistor Testing

The air conditioner repair help you need from Repair Clinic

Need some assistance in locating and uninstalling a window or wall air conditioner’s thermistor? Repair Clinic’s “Video & Articles” library has the repair help you need. Enter the unit’s full model number in the search bar to find model-specific diagrams identifying the location of every part as well as step-by-step guides and videos showing how to replace those parts, such as this one taking you through replacing a condenser and outdoor air thermistor on an LG Air Conditioner (Model LW2217IVSM), or this video demonstrating the correct way to install a new fan motor on a Frigidaire Air Conditioner (Model FFTA0833U10).

Find genuine OEM air conditioner parts from Repair Clinic

Repair Clinic also makes it easy to find the right thermistor, fan motor, fan blade, blower wheel, run capacitor, or air filter that matches the specific unit you need to repair. Just enter the full model number of the air conditioner in the Repair Clinic website search bar to see a complete list of the genuine OEM parts compatible with your model, from top manufacturers like Admiral, Bryant, Carrier, Frigidaire, GE, Goldstar, Goodman, LG, Lennox, Payne, Rheem, and WeatherKing. You can narrow that list down using the “Part Category” filter (example: “Sensor & Thermistor”) followed by the “Part Title” filter (example: “Thermistor”) to locate the exact part you’re looking for. By providing you with quality replacement parts plus the know-how to fix the AC unit yourself, Repair Clinic is a valuable repair partner.

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