Car AC blowing hot air

Car AC Blowing Hot Air? Here’s Why And What To Do About It

Nothing is more frustrating than finding out that your car’s AC system just stopped working on a hot summer day. You are walking to your car and the pavement is so hot your shoes are sticking to the asphalt. All you want to do is jump in, crank the AC, and get out of the heat. But when you flip the switch and turn the dial down to cold, nothing happens. There’s nothing but hot air coming out of the vent. Luckily, if it’s your case, we’ve created this article to help you understand why your car AC is blowing hot air and what you can do about it.

Automotive AC System Basics

Air conditioning was first installed in cars in 1933 and by 1969, about half of the new cars sold in the United States came with factory-installed AC systems. Aftermarket AC options were also available back then and still remain a popular add-on for people who drive older cars not originally equipped with AC. However, both factory and aftermarket AC systems can stop working at some point and cause hot air to come out of the vent.

As opposed to popular beliefs, air conditioning systems don’t blow cold air. Instead, AC systems remove heat and moisture from the air, lowering the temperature at the same time. If your car AC is blowing hot air, you can safely assume that one of the components is faulty.

Types of Refrigerant

In the old days, cars had AC systems that used a chemical called R-12. Subsequent researches then showed that R-12 depleted the ozone layer and was not used in cars produced after 1994. R-12 was an excellent refrigerant that provided a massive temperature drop and was otherwise safe to use until it was discovered that CVC’s, of which R-12 is one, deplete the ozone.

Today, automotive AC systems use R-134 instead, which is a lot safer while still being harmful to the environment. Between the early 1990s and the early 2000s, manufacturers also used R-134a, commonly known as Freon. DuPont marketed R-134a as Freon and it quickly became the industry standard. R-134 and R-134a are not as efficient as R-12 was but are still significantly safer for the environment.

Converting an older R-12 system to a modern refrigerant is possible, but could be expensive.

Car AC compressor

Air Conditioning System Operation

Your AC might seem like a complicated system, but once you understand the basics, it is pretty simple to find problems. The best way to understand how your AC works is to look at the different components that make up the system. No matter what type of car or truck you drive, all car air conditioning systems are similar. If your car AC is blowing hot air, there are a few places you can check for problems before paying a shop for repairs.

AC Compressor and Clutch

The most obvious part of your AC system you will see under your hood is the AC compressor. This is the large pump at the front of the engine. Most cars have AC systems that are belt-driven. At the front of the compressor is a pulley with a built-in clutch. The clutch makes it possible for your AC pulley to only engage when you want the AC on. That way, the AC pump isn’t constantly turning, seriously reducing gas mileage.

The AC compressor creates pressure in the system and causes the refrigerant to liquefy. In a liquid state, the refrigerant is passed through the condenser, filter, and dryer where heat is removed. The refrigerant then passes through an expansion valve that returns it to a gaseous state and is returned into the evaporator while a fan blows the cooled air into your car.

AC Lines

Your AC system uses two lines to transfer the cooled refrigerant to the inside of the car. One of these hoses is the supply line, the other is called the return line. You will notice that the lines have ports on them with plastic caps. These ports are fill-and-check points you can use to recharge, drain, and test the system.

AC Evaporator

When the compressor pumps refrigerant into the evaporator, it is in a gaseous state. The gas boils, absorbing heat and moisture from the air. Inside your car, a small radiator-like device allows warm air to blow across the cooling fins of the register to create that wonderful, cold air you are counting on.

AC Condenser, Filter, and Dryer

You may have noticed that your AC-equipped car has two radiators. The larger radiator cools your engine, while the smaller radiator, called a condenser, cools the refrigerant used in your AC. The AC condenser will typically have a cylindrical part attached to the side or a little to the side but still connected to it. This component is the dryer and it makes sure any moisture trapped in the system is removed. Most dryers include a built-in filter that traps solid particles that can cause serious problems with your AC.

Your filter and dryer should be replaced any time the AC system is opened for service.

Auto mechanic servicing AC system

Common Reasons Why your Car AC is Blowing Hot Air

When your car AC is blowing hot air instead of cold, you might think that you are in for expensive repairs. Sometimes, fixing a broken AC can be expensive, but not always. Many repairs are simple to make on your own.

Most air conditioning problems are a result of the system failing to maintain pressure. Without pressure, the refrigerant cannot be liquified and can’t transfer heat properly. When there’s a leak in the system, the refrigerant will also evaporate into the atmosphere. Once the pressure reaches a certain minimum, the compressor will simply stop working. Furthermore, once air can enter the system, the humidity it contains will also fill up the AC dryer and prevent it from doing its job. All those conditions will make your car AC to blow hot air.

Here are some of the most common causes:

Damaged Lines and Seals

A common reason that AC systems lose pressure is because of damage to the lines or the expansion valve. When an AC line cracks, the refrigerant escapes, and the system is no longer able to build up the pressure. This will cause the AC to stop working and could even damage the compressor.

Seals also tend to dry up with time and start leaking. When trying to find the source of a leak, always pay special attention to the connections of AC lines.

Clogged Condensor, Filter, or Dryer

Just like your cooling system’s radiator, when your AC condenser is clogged or damaged, it will stop working efficiently. Most of the time, a condenser with clogged cooling fins will not work very well but can be cleaned using a high-pressure washer.

Over time, the filter and dryer assembly can become clogged with moisture and debris and will stop functioning. Wet air does not cool as well as dry air, so your system will not bring down the temperature inside your car as much as it should. You can’t usually repair or rebuild your AC filter or dryer and must replace it instead.

The Compressor Clutch

A faulty compressor clutch is one of the most common causes of an AC system not working. The clutch is installed over the compressor pulley and is the component ultimately activating the system. When the driver turns the AC on, an electrical signal is sent to a coil of copper wire in the clutch. This creates a magnetic field that pulls the clutch onto the pulley shaft and bearing and engages the compressor. When you turn the AC off, the clutch disengages and allows the belt to turn without powering the compressor.

AC expension valve

Unfortunately, the compressor clutch can fail at any time. As with any other electrical component, if the circuit becomes open or short, it will stop working. When it happens, you can either rebuild or replace the clutch on your AC compressor.

Faulty Expansion Valve

The expansion valve is the component turning the refrigerant from liquid to case, allowing it to absorb. A clogged or stuck expansion valve will cause the AC to blow hot air.

Once again, once it’s broken, there’s not much to do but to replace it.

Loss of Refrigerant

Your AC system is a closed system and should never need to be refilled. The only time that you should need to recharge your AC is if you have a leak. Some people believe that you must periodically recharge or “top-off” your AC system. Not only can this be dangerous for your AC system, but it’s also a waste of money. If your AC system is not holding the correct amount of refrigerant, there’s a leak somewhere. While refilling it may cause it to work for some time, it will only do so until the pressure goes down again.

Keep in mind that the leaking refrigerant will also escape in the atmosphere which is harmful to the environment.


If your car AC is blowing hot air, it’s always handy to be able to replace the faulty component yourself. However, finding which components needs to be replaced is a whole another story and we’ll have to keep that for another post.

Luckily, some easy pressure tests can be performed at home. On the other hand, each AC system works with specific pressure thresholds. If you plan on troubleshooting your own AC system, get your hands on your car’s repair manual to find out the correct values for your specific car model as the right test procedures.

It’s also important to mention that in some states and Canadian provinces, a certification is required to handle, recuperate and fill up AC refrigerant. Luckily, you can still bring your car to the repair shop, have the refrigerant out, do the repair yourself and have it filled back up when it’s done.


About Derek F

Derek grew up in Southern California and started working on cars when he was a child. He learned from his father and grandfather how to make basic repairs and maintain cars correctly. Derek rebuilt his first engine at 15 years-old, beginning an automotive career that took him to many interesting jobs. Derek has worked as an automotive detailer, managed parts warehouses and auto parts stores, and worked as a mechanic for several years doing brake and suspension work. While pursuing a bachelor’s degree in government journalism, Derek worked for an auto museum where he started to write about cars. Today, Derek uses his expertise gained from many years of practical experience to help educate DIYers and share interesting knowledge about various types of automotive repair and service. Writing about cars helps fund his numerous classic car restoration and customization projects.


  1. Avatar

    We experienced a lot of hot air in our car the other day and I was wondering if this was just a result of the recent heat waves or if this was a problem with the car itself. I appreciate you talking about leaking refrigerant and how it’s something that should be a cause for alarm because my wife told me she found “strange-looking water” under the car the other day so this might be what she was talking about. I’ll make sure to find an auto service that can help us get this checked out as quickly as possible. Thank you for the article!

    • Avatar

      Glad you found the post helpful! There are a few tips on how you can self-diagnose the issue. Hope you get it fixed as we enter summer months!

  2. Avatar
    Victoria Addington

    Thanks for listing the common reasons for car AC damage. Apparently, I’ve noticed that my car ac has been blowing hot air in the past weeks. I had no idea that a broken compressor clutch is one of the most common causes of a faulty system. With that said, I shall then find repair services here in town to diagnose the problem and get it fixed.

    • Avatar

      Good idea on getting it diagnosed by a shop, it should help you determine the issue. Let us know the outcome!

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