Why Is My Car AC Blowing Hot Air?

“When I turn on my AC, it blows warm and doesn’t blow cool. Why is my car AC blowing hot air?” This is one of the common questions our experienced mechanic Sue has run into in her career. Learn about the AC system and what you can do if you have a car air conditioning blowing hot air.

Why Is the AC in My Car Blowing Hot Air and How Do I Fix It?

Mechanic under the hood reviewing an AC system blowing hot air into a car

The air conditioning system in your car has 5 major components:

  • Compressor
  • Condenser
  • Evaporator
  • Receiver driver
  • Expansion valve or orifice tube

5 Major Components of the AC System in Your Car

1. Compressor

The compressor is driven by a drive belt. It may have a separate belt or the pulley, and it may be driven by the serpentine belt. You’ll find a high and low side AC line, each designated to carry the refrigerant in liquid and vapor form.

More on how to inspect an AC compressor

2. Receiver-Drier

AC lines travel to the receiver-drier, which is a canister to remove moisture from the refrigerant so it doesn’t seize the pump.

3. Evaporator

Lines travel to the firewall with a low and high side to the evaporator, which works similar to the heater core and allows the cool air to flow into the cabin.

More on how to diagnose an evaporator

4. Expansion Valve or Orifice Tube

In this article’s video, there’s an expansion valve is in this system, and it’s there to tell if you have high or low pressure for the compressor. Cars will either have an expansion valve or orifice tube, and not both.

5. Condenser

The condenser is at the front of the vehicle. Because of this, it can be damaged by outside elements and debris.

More on how to diagnose a condenser

How to Diagnose and Fix the AC System If the AC Is Blowing Hot Air in Your Car

Steps to diagnose and fix the AC system

  1. Use an AC Recovery Machine or Have a Professional Drain Refrigerant

    When working on the AC, it’s important to to have an AC refrigerant recovery machine. These machines can cost thousands, but if you don’t own one you can still work on the AC system yourself. You’ll need to go to a shop that has one, pay for a service so the system can be evaporated, and then bring your car back home where your can work on it.

  2. Use a Headset to Diagnose the AC System

    If you want to diagnose the AC system yourself, you can buy the headset without the recovery machine. These can cost under $100 and all you will need to do is connect it and run the car. The headset will have blue and red gauges for the high and low pressure side. These gauges can tell you a lot about what is going on with the AC system.

    Check out the video at 3:06 for a demonstration on how to understand and use a headset. In the example, the gauge on the low side stays steady and reads the refrigerant level as full because the system has yet to be turned on. When the AC is turned on and the compressor runs, the gauge will fall on the low side and rise on the high pressure side. If the AC has a problem like the driver turning it on and it turning off, or turning the AC on and receiving warm air, the high side would rise much higher, like near the 300 or 350 range, indicating there’s a problem. When the clutch in the AC compressor engages and disengages, each gauge for each side will rise and lower. When the clutch disengages, the cold side should rise and the high side should drop, and vice versa. As it cycles the refrigerant, it’s a bad sign if the gauge rises past 300 and up.

    You my find that the system is charged, works fine, but around 10 minutes after turning it on, it blows warm air and you find the system is not circulating. You’d know you have refrigerant in the system because it’s cold. The system can’t go cold to hot with no refrigerant. If you have refrigerant and it’s not leaking, the condenser is the easiest part to check is condenser.

  3. Spray the Condenser with a Garden Hose to Clean It

    The refrigerant sits in the AC condenser. Air travels through the condenser, which has cooling fins, to cool down the refrigerant. Because of its placement, the condenser picks all kinds of debris from the road, like dirt and sand. It can clog over time, and this can stop the system from cooling.

    If you live in an area with dirt roads, for example, washing the condenser with a garden hose can prevent the condenser from failing. A pressure washer is too strong. The condenser is made of aluminum and a pressure washer could bend the fibers inside. If you’re doing this with a headset, you’ll see the high side drop. Once you’re done rinsing, let it stay down, and this may fix the problem. If the gauge rises after rinsing, the water is only temporarily cooling it.

    If the AC condenser is hot, it can damage the compressor, which is a tedious job and will require you to remove the refrigerant. If replacing the compressor, you’ll need to flush the system, and then replace the condenser with a new receiver dryer. Receiver dryers usually last around 3 to 5 years because of the moisture in the salt. If you only replace the compressor, you could have a partially damaged condenser, which fill cause future problems.

  4. Make Sure the Cooling Fans Are Working

    The AC and coolant fan can also cause a cold to hot AC. The fans cool the coolant traveling through the radiator and the refrigerant traveling through the condenser. The AC fan might not turn on and that can overheat the AC. The condenser needs a fan to cool down the refrigerant.

  5. Do Not Use an AC Recharge Canister

    We don’t recommend using an AC recharge kit as that can cause more problems with the ac system, like adding too much refrigerant.

Find out why the AC in your car is blowing hot air yourself with quality auto parts at 1aauto.com

More Articles and Videos to Diagnose the AC System

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Why Is the AC in My Car Blowing Hot Air? - Expert Tips - 1A Auto
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Why Is the AC in My Car Blowing Hot Air? - Expert Tips - 1A Auto
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If you want the answer to the question, why is the ac in my car blowing hot air, check out these tips from the expert mechanics at 1A Auto
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