Why is My Window AC Not Cooling?

Nobody likes a broken air conditioner! When the temps are rising outside, it is essential to have your window AC unit working properly for you. In this article, we want to help you answer the question, “Why Is My Window AC Not Cooling?”

Five Common Reasons Why your Window AC is Not Cooling

These are the top reasons why your window AC unit may not be pumping out the cold air like it’s supposed to. They are arranged from the easiest do-it-yourself fixes to the type of issue you may need to take to the professionals. We also cover an AC problem that could indicate that it’s time for a replacement unit.

Your Window AC Is In the Wrong Mode

Your window air conditioner not cooling could be for one simple reason. It might not be in Cool mode. Window air conditioners are designed to have multiple modes, and each mode serves a particular function. Not every window AC is built the same way, and so there is some variation of which modes your AC might have. However, almost all window air conditioners have these three modes: Cool Mode, Fan Mode, and Dry Mode.

Here is a brief explanation of what each mode does. Know your modes – and get the most out of your investment in a window air conditioner!

Window AC Models

Cool Mode

When your AC unit is on Cool Mode, it will be removing heat in the room until it reaches the set temperature of the AC thermostat. As a bonus, it will dehumidify the air too, making it more comfortable, less “muggy.” Once it reaches your desired room temperature, it will work to maintain this temperature in the room until you adjust the thermostat to a different temperature or change the mode on the unit.

If your window AC is not cooling, this is the first thing you should check. Is your window AC in Cool Mode? If not, try putting it in Cool Mode and see how that changes things. If it is in Cool Mode, and is still having trouble cooling your space, then there is probably another issue going on, and you will need to keep hunting for the source of the trouble.

Fan Mode

If your window AC is in Fan Mode, it is basically functioning as a simple household fan would. In fan mode, the unit does not blow any cold air, but instead circulates the air that is already in the space to create good air flow. This mode is perfect for a fresh spring day, where the outside air temperature is ideal, and you just want that extra air circulation through the house.

Dry Mode

Most newer window air conditioners have this mode. Another name for it is Dehumidifier Mode, and, as you might have guessed, its job is to dry out the air! When your window AC unit is on Dry Mode, it is working to remove moisture from the air only – it does not cool the air. This mode is ideal for a cool, humid day where everything just feels damp, and your main goal is to dry things out, not necessarily cool your house.

Now that you are familiar with each mode and what it does, check to see if you are in Cool Mode before proceeding further! The answer could be this simple fix.

Your Window AC Filter is Dirty or Clogged

And the fins on the back might be dirty too.

The air conditioner filter is something that is meant to be cleaned from time to time. If your filter is dirty or clogged, it is probably restricting the air flow and preventing your AC from cooling at its normal capacity. In addition, if the filter is very dirty, it could cause frost to form on the evaporator coils, which further inhibits good air flow. The best fix for this is to replace the filter and remove any material that might be clogging the filter area. If you notice any dirt build-up on the evaporator coils, you can clean them with warm, soapy water.

This is one of the most common AC problems, so be sure to check the filter and the fins/coil in back if your window air conditioner is not cooling properly!

Your Window AC is the Undersized for your Room

Not all window air conditioners are equal in the amount of heat they can remove per hour, or to say it another way, how much cool, dry air they can deliver. It may be that your window AC isn’t cooling because it is too small for the space.

Window AC cooling power is measured in BTUs, which stands for British Thermal Units. Most window air conditioners need 20 BTU per square foot of space to be cooled. For example, if you are trying to cool a 500 square foot room with an average ceiling height, you will need a window AC unit with 10,000 BTU.

Before assuming that your window AC is broken, make sure you check to see how much cooling power it has by finding out the BTU rating! You might just be using an air conditioner that is simply too small to cool your space. If this is the case, you can either size up and replace your current window AC with a unit that has a higher BTU rating, or you can invest in a second unit to work together with your current one.

Either way, it is always a good idea to check and make sure that your window AC has the BTUs to handle the amount of space you are wanting it to cool. This will not only ensure that you have an AC that will actually be able to cool your space, but also keep your window AC from running electricity and raising your bill without effectively cooling your home.

Is too big better? No. You don’t want to use a window AC unit that is much larger than what you need. This will:

  • Cool the room quickly and shut off before it has removed much moisture. The result will be a room that is cool and clammy
  • Unnecessarily burn up electricity and raise your monthly bill.

You want to find the right size window air conditioner for your home in order to maximize the cool air and minimize the electricity costs; not too small, and not too large!

If you are having trouble calculating what size air conditioner you need for the space you are trying to cool, use our AC BTU Calculator for the purpose.

It is an easy and valuable tool to use to check on your window AC size – if it is right for the room size.

If you decide to buy a new window air conditioner, definitely use the Calculator to ensure you buy one that is a “perfect” size for the space.

Your Window AC Has a Broken Thermistor

Window air conditioner units have something called a thermistor. It monitors the temperature of the area to be cooled, and sends signals to the compressor to turn the cold air on or off according to the temperature of the area [e.g., your living room.] If the thermistor senses that the area has reached the desired temperature, it will send a signal to the compressor to shut the cool air off.

Likewise, if the area is still too hot, a properly functioning thermistor will signal the compressor to keep blasting the cold air. One possible cause of your window AC not cooling is a broken thermistor. To locate your window AC’s thermistor, you will need to remove the outer case of your unit to expose the electronic control board. Once you have located the control board, the thermistor is a small part with a glass bulb on it. Inspect your thermistor for signs of wear-and-tear, taking a careful look for any disconnection or damage.

If your thermistor has any signs of damage, you should replace it. You can do this yourself, but make sure to replace your old thermistor with a new one that is approved by your window AC’s manufacturer! If you are more comfortable leaving the air-conditioner dissection to the professionals, don’t worry; they’ll know exactly how to diagnose and take care of your broken thermistor. Soon you’ll have that cool air flowing through the house again!

Your Window AC Has a Broken Compressor

The compressor is one of the most important parts of an air conditioner. It works like a pump to circulate refrigerant that captures heat inside your home and dumps it outside. This makes the cold air flow, and without a properly functioning compressor, the air conditioner can’t work properly. A sign that your compressor may have failed is if your AC is blowing warm air, even in Cool Mode, and when the filter and fins are clean, and there are no other apparent issues with the unit. If your compressor is broken, it is probably time to consider a new window air conditioner.

Is it worth it to replace the compressor? Probably not. Usually, if a compressor fails, it is because the air conditioner is an older unit. The compressor is an expensive part to replace, so it is likely more cost-effective to invest in a new window AC, rather than a replacement compressor.

When to Replace your Current Window AC (need)

Unfortunately, window air conditioners are expensive to repair relative to the cost of a new one. Most repair shops charge $50 to $75 just to diagnose the problem. Then there’s the cost of parts. With a bill of $100 to $225 in many cases, you are more than halfway to a new window air conditioner. 

When we’re asked when to replace your current window AC – when do you need to replace it – our general answer is this: The older it is, the less you should put into repairs. Spending $100 or $150 to repair a room AC that is less than 5 years old makes sense. Spending that on one more than 8 years old doesn’t make sense, in our opinion. In between, it depends on your budget and circumstances. For example, if you live in an apartment but plan to move in a year or two to a home – probably one with central air conditioning, then repairing the unit is a cost-effective decision rather than replacing it.


Hopefully you were able to diagnose and fix the reason your window AC isn’t cooling. However, if you suspect it is broken and you plan to buy a new unit, see our Window AC Buying Guide with comprehensive information that will help you to choose the right size window air conditioner for your space.

We hope this guide is helpful and that you are able to get cool air flowing through your home again soon!

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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