Why is My Car’s A/C Blowing Hot Air?

car AC blowing hot airThe air conditioning system in your car is designed to cool down the air in the cabin of your vehicle to keep you comfortable on a hot day.  When you only get hot air instead of cold air, it can be frustrating and extremely uncomfortable.  In this article, we’re going to look at how your Car’s A/C creates the cold air so you can more accurately diagnose what’s going wrong.

The cool air is created by blowing air from either outside your car, or from the cabin over cold coils.  This cools down the air before it gets blown out of the vents into your car to keep you comfortable.  Most systems are designed to blow very cold air, between 32 and 40 degrees!  These cooling coils, called the evaporator, is where the actual cooling of the air happens.  These coils are cooled by cold refrigerant flowing through them.

Most cars today use a refrigerant called R-134a which is a special refrigerant designed to work at the temperatures and pressure standard in a car’s A/C system.  As the cold refrigerant passes through the evaporator it absorbs the heat in the air which warms up the refrigerant.  This warmer refrigerant now needs to be cooled back down so your A/C system can continue to operate.  This is accomplished by first compressing the refrigerant to a high pressure which also heats it up to a high temperature.

This hot refrigerant is then cooled down by running it through the condenser.  The condenser is cooled by ambient air in the front of your car just like the coolant in the radiator is cooled.  On most cars the condenser is actually in front of the radiator and gets cooled first to make sure your A/C system gives you consistently cool air.  This high pressure, now cooled, refrigerant is then allowed to expand quickly as it passes through an orifice or expansion valve.  The expansion valve is a simple device that takes advantage of the fact that quickly expanding liquids and gases get cold.  The process of liquids and gases getting cold during expansion is a scientific phenomenon you can prove by using an entire can of spray paint in one sitting and seeing how cold the can gets.

Now the cold, low-pressure refrigerant can return to the evaporator to cool it and along with it, more air keeping your car comfortable.  There are a few other components in the system like temperature and pressure sensors, hoses and the dryer or accumulator.  The dryer is where any moisture is removed from the refrigerant, the refrigerant is filtered to make sure it all enters the compressor as a gas, and the oil for the system is equally distributed through the refrigerant passing through.

car AC blowing hot airIf your car’s is blowing hot air then one of these systems isn’t doing its job properly.  Since there are two components that require air to be moving through them, the evaporator and the condenser, it is possible one or both of these are clogged and not getting the proper air flow.  Check the front of your vehicle for debris, the engine cooling fans for proper operation, and your cabin air filter for debris and dirt.  The refrigerant in your system also needs to be able to flow properly through the system.  Check your compressor for proper operation, clogs in your system and the proper amount of refrigerant.  All of these can be checked by observing the pressures around the system both before and after the compressor and all the other components.  This can be done with set of A/C gauges to measure both the high and low pressure sides of your A/C system.

If you find unusually high pressure before any particular components and unexpectedly low pressures after them, there is a good chance there is a clog in that component.  If this is the case, you will most likely find the clog in the accumulator or the condenser.  If you don’t have a large enough pressure increase across your compressor you may be in need of a new AC compressor.  If you have lower than expected pressures on the low pressure side of your system, may simply have a low refrigerant level.

If you do not have enough refrigerant in your car’s A/C system the compressor may not be able to raise the system pressure high enough for the expansion valve to be effective in cooling down the refrigerant.  Due to the high pressures of your air conditioning system and the volatile nature of the R-134a refrigerant used, it is not uncommon for some of the refrigerant to leak out over time causing a low pressure condition in your system.  If you discover this to be the reason why your A/C is blowing hot air add Red Angel A/C Stop Leak to your vehicle’s air conditioning system to seal any leaks no matter how small they are before you recharge the system.

Red Angel A/C Stop Leak One Shot comes in an easy to use aerosol can that can be installed quickly and easily without any tools and also contains 2.5 ounces of R-134a to help bring your system back up to normal operating pressure.

Red Angel AC Stop Leak & Conditioner

To learn more about Red Angel A/C Stop Leak Aerosol visit our product information page here: A/C Stop Leak Aerosol

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Pictures provided by:

car_ac.jpg – By Mark Coffey – Licensed by Creative Commons – Original Link
cabin_air_filter.jpg – by Donar Reiskoffer – Licensed By Creative Commons Via Wikimedia – Orginal Link


32 responses to “Why is My Car’s A/C Blowing Hot Air?

    1. Dave-

      All of the email addresses work. We receive and send multiple emails daily. I will email you now directly.

      Make you you are using the ending @gobdp.com rather than @bluedevil.com



        1. Roy,

          Thanks for your question about your CLK430. More often than not, when a car is blowing hot air only at a stop, the problem is with the electric cooling fans. Those fans should kick on when you turn your air conditioning on to keep enough air moving through the condenser while you’re stopped. If you don’t see or hear these fans try checking electrical connections, fuses and relays. If the fans are coming on, the problem is most likely with your air conditioning system and you should have the pressures around the system checked to see what components are malfunctioning or if you have a leak.

          Thanks again for your question!
          -BD Auto Pro

  1. i have a 97 buick park avenue put 2 cans of freon in it and the next day it was blowing hot air again do the stop leak fix big leaks as mine thats not even lasting a day

    1. Dre-

      It sounds like your leak is a little bit too extreme for what the Red Angel is intended to fix. Your best bet may be to take the Buick into an A/C specialist to be repaired.

      Thank you!


  2. Hello,

    I have problem with my ford cougar 1999 A/C Does not matter what settings is on, it always blows cold, freezing cold air. Its like the setings stuck in cold air setings. When i change settings, i hear the valves moving, changing and air flow is changing too, but it always blows only cold air. I checked the pressure of low and high pressure valves and they are correct (Low just below 50 and hig below 200). When i start car and turn a/c on to warm or cold settings, the pipes around engine gets frozen white. Antifreeze levels are full to the max point. I have ordered thermostat, will try to change it, but not sure if that will help. I never hade this problem before, so have no clue what to check. The compressor looks like it working as it should be. Any ideas what can be wrong?


    1. Ged,

      Thanks for your question about your AC. It sounds like the problem is with your heater not your AC. Even if your AC is on, your heater should still warm the air back up. Your problem could be in your ventilation system. Even if the motors are moving the flapper door that should direct air over the heater core may not be moving. To check this, you’ll have to dig into your vent ducts and test that flapper door. The other problem is that your heater core could be clogged, or your thermostat stuck open. If your thermostat is stuck open, you’d notice the temperature staying very low in your car for a long time after starting. If your heater core is clogged, try using BlueDevil Radiator Flush (available here: http://store.gobdp.com/radiator-flush-00204/).

      Thanks again for your question!
      -BD Auto Pro

  3. I have a 2006 Skoda Octavia 1.9TDI. On hot days, when i get into the car and turn on the AC, nothing happens. I just get warm air flowing out of the vents. After 20 or thirty minutes of driving, when i turn of the AC and turn it back (1 minute delay), the AC starts working. Also, it doesn’t cool on such hot afternoons unless the blower speed is set to position 2 or 3. I rarely use position 4 cos its noisy. However on cool evenings and mornings, the AC blow cool air instantly. The refrigerant is fully recharged. My condenser is a new one. .
    Sometimes when i turn the blower speed down to 1 or adjust the temperature (from 18 degree Celcius to a comfortable 22 degrees Celcius), the AC cuts off and doesn’t come on again.
    I have checked the cabin air temperature sensor and it works fine and i have had to change it to see if things will improve but it didn’t change anything.

    1. Cyprian,

      Thanks for your question about your Skoda Octavia. There are a few possible problems you might be having. First, you might have a faulty pressure switch in your system. On warm days the pressure in your AC system is higher and if your pressure sensor is faulty and reading too high it might trip the system off on warmer days. Second, it could be that your AC clutch is malfunctioning and on warmer days, or when more load is on the system it fails to engage the AC compressor which would cause your system to not operate correctly. Lastly, there could be a clog in your system that only causes problems on warmer days when pressures are high.

      Thanks again for your question!
      -BD Auto Pro

  4. I have a 2004 Chevy Tahoe that intermittently blows hot air when the A/C is on. I hear a clicking sound for a few minutes and then the hot air comes out. Just by accident, I found that if I pull over and turn the car off and then turn it back on, the A/C resumes. Any ideas of what is causing this problem?

    1. Dave,

      Thanks for your question about your Chevy Tahoe. The problem you’re having sounds like it’s actually a problem with your ventilation control unit. The clicking sound you’re hearing is probably the air blend doors moving or trying to move behind the dash until the blend door moves to the “hot” position directing the air over your heater core so it comes out of the vents warm. When you shut your car off for a short time it resets the unit. You can try pulling the fuse for heater control unit for a few minutes, then replace it and turn you truck on the set your ventilation control unit into calibration mode. If that doesn’t fix the problem then the servo motor or the door placement indicator is probably bad sending a bad signal to the control unit.

      Thanks again for your question!
      -BD Auto Pro

  5. hi i have a 2007 ford freestyle. The A/C blows hot/ warm at idle and driving. I can hear the compressor turn on when I hit the button. The low pressure port is 100 lbs which sounds high to me and when I checked the high pressure port a bunch of yellow greasy liquid came out any ideas on how to solve this? I am a DIY guy but have little experience with A/C and trying to learn.

    1. Eugene,

      Thanks for your question about your Ford Freestyle. Depending on if your compressor was running or not, 100psi does sound high for your system. To check the high pressure side you’ll need a set of mechanics AC gauges. With a set of gauges you should be able to tell if your system is overfilled with refrigerant, if the compressor is working properly and if there is a clog in your system which are the three most likely problems you’re having based on your description.

      Thanks again for your question!
      -BD Auto Pro

  6. I have 2008 Nissan Altima. I put Freon in and it worked for like a month, then hot air again. I took it to a shop and they fixed a leak and it worked perfect for 5 days now today it was hot air again after 45 mins it was finally cold. Could this be a more serious issue like needling to replace the condenser and compressor?

    1. Jana,

      Thanks for your question about your Altima. Based on how long it takes for your AC to get cold, it sounds like you’ve got a clogged component in your system, possibly the expansion valve or the dryer. It is possible that your compressor is going bad as well, but less likely. To check for all of these problems you’ll need check the high side pressure in your system. If it is too high then you’ve most likely got a clog, and if it is too low then your compressor may be failing.

      Thanks again for your question!
      -BD Auto Pro

  7. I have a 2003 Chevy Silverado my AC was blowing cool air not cold up until a couple days ago. I bought some air conditioning recharging fluid put the gauge on there so that it was a little low. Then I added some until the gauge read in the blue where it should be but now my system only blows hot air I have no clue why. Somebody help me please.

    1. Levi,

      Thanks for your question about your Chevy Silverado. The gauges on the refill AC cans are not vehicle specific so it is possible that your system is now overfilled with refrigerant. That could cause it to trip off on over pressure or for your dryer to not be able to keep up with the extra refrigerant which would also cause your system to shut down.

      If you use a recharge can that has a particulate leak stop agent you could also have a clog in your system which would keep it from operating properly. Lastly, you could have a large leak in your system and even though you just added refrigerant, it’s all leaked out so you could try checking the low side pressure with the gauge again just to verify you don’t have a large leak.

      Thanks again for your question!
      -BD Auto Pro

  8. I have a 2007 Toyota Camry. My air conditioner is acting crazy. It blows cold air when it’s not real hot outside like 75 degrees. Then when it gets hot outside it blows warm air. Mechanic measured freon and it was OK. Do you know what the problem could be?

    1. Dianne,

      Thanks for your question about your Camry. Based on your description your system could be over filled with refrigerant or you may have a faulty pressure or temperature switch. On warmer days, your AC system pressures get very high so if it’s overfilled it could be tripping off on an overpressure causing the warm air and if you have a bad pressure or temperature sensor it could be inadvertently shutting your system down because the sensor is reading incorrectly.

      Thanks again for your question!
      -BD Auto Pro

  9. I have a 2000 buick Century when you turn on the a.c. it blows warmish air but when switched to heater it only blows warmish air and doesn’t heat up either, What could cause this? I bought a.c. recharge cause it was a little low and that didn’t help at all. Checked all other liquids and they are where they should be also. Please help

    1. Tiffany,

      Thanks for your question about your Buick Century. It sounds like your problem is with your ventilation system. When you change the temperature switch it should move an air blend door which directs air either over your AC cooling coils or your heater core. If the motor that moves this door, or the door itself is damaged then it would cause the same temperature air to blow out of the vents no matter what. unfortunately to get to the air blend door to check it you have to remove a significant amount of your dashboard and air ducting.

      Thanks again for your question!
      -BD Auto Pro

  10. My air conditioner was blowing cold air. We replaced the thermostat in my 2002 Ford Taurus and the air flow coming from the air conditioner immediately began blowing warm air. We checked the freon and the compressor, and everything seems fin.

    1. Missie-

      Thank you for asking about your Ford Taurus. Based on your description, you may have some kind of a restriction in the system, not allowing proper circulation. We recommend bringing the vehicle to an A/C specialist for an A/C Flush. Feel free to contact our technical support line at 888-863-0426 with any other questions.

      Thank you!


  11. My 2010 Toyota corrolla ac was running fine…yesterday when I was at my mechanics for an air filter change I asked him to check my temperature in my car since it is almost 7yrs old and I never had a recharge..he said it was at 62 degrees and that I probably could use a lb of freon..so I let him do it and it needed less than a half of pound…I took the car home literally around the corner and today my cars ac is putting out hot air…what did they do to my ac…

    1. Denise,

      Thanks for your question about your Corolla. Your mechanic was correct that 62 degrees is a bit warm for air conditioned air. Hopefully, by adding more refrigerant your system simply got over filled and has shut off due to a high pressure switch being triggered. You can try checking your high pressure side pressure to see if it’s outside of the normal operating range to prove that is what’s happening.

      There are some other remotely possible things that happened like the fill valve got stuck open after he removed the AC machine, or that it was contaminated refrigerant. Hopefully if you take your vehicle back to that mechanic he will be able to check his work and determine what the problem is.

      Thanks again for your question!
      -BD Auto Pro

  12. i need to know why my ac blows warm air sometimes but not all the time. 2006 honda accord…its driving my wife crazy!! I DROVE the car the other day and it was very cold!! but later that night it started blowing warm air!! it has been doing that all summer. please help

    1. TGreene,

      Thanks for your question about your Honda Accord. Usually, with intermittent problems like you’re describing it is an electrical issue. You might have a bad connection for the signal wire to your compressor, a bad pressure or temperature sensor somewhere, or even a problem with your electric fans. Chasing down electrical issues can be a frustrating and time-consuming process, but if you’re willing to try it, start by reading our article about that here: https://gobdp.com/blog/diagnosing-electrical-problems/.

      Thanks again for your question!
      -BD Auto Pro

  13. I have a 2011 ford fusion. I noticed issues with the ac when i would turn it on i can hear everything come on. Air would be blowing cold for 2 seconds and then it switches to hot air. And i mean hot like a blow dryer in your face. We thought it was the evap sensor so we pulled the dash and changed it. It worked last night. I got in my car today and it’s not working again. We checked the freon levels and it is normal. What are we missing? What could it be? Please help!

    1. Amanda,

      Thanks for your question about your 2011 Ford Fusion. Based on your description of how hot the air blowing out is, our guess is that you’ve actually got a problem with your ventilation system. There is a door in your ventilation system that directs the vent air either over the cooling coils or the heater core and it sounds like that is malfunctioning in your car and it’s switching to the “heat” position when you don’t want it to. This may be a problem with the servo controlling the door or with the computer module in your ventilation system. You can try checking the servo for operation but you may have to take your car to a Ford dealership to have them run a diagnostic on the ventilation module.

      Thanks for your question!
      -BD Auto Pro

  14. I have a Nissan xTrail T32
    when outside temp is below 10 degrees, I have my A/C on all the time, inside temp set to 20, the air out of vents is hot (feeling above 30 degrees) and during the 30 minutes drive does not change whatever the inside temp I set, either 17 or 25, wrong or normal based on sensors checking the inside temp, to me not as inside temp after 30 minutes ride is already high, quite hot so sensors should pick it up and adjust the air temperature blowing out of the vents as I have the inside temp set still to 20. What is the point of setting inside temp when it does whatever it does. It is automatic aircon but I do not use auto mode, I set it myself manually. Thanks for reply

    1. Marian,

      Thanks for your question about your Nissan xTrail. Based on your description it sounds like your automatic HVAC controls are malfunctioning. The problem could be coming from a bad sensor or a bad HVAC module. Unforutenetly, the only way to test those components is with a special computer from a Nissan technician so without replacing parts that may or may not be good, your best bet is to take your vehicle to the dealership.

      Thanks again for your question!
      -BD Auto Pro

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