Q: Why Is My AC Blowing Warm Air?

asked by on November 20, 2015

Why is my AC blowing warm air?

There are quite a few reasons an A/C system will cease to provide you with cold air. Air conditioning systems are one of the more complex systems on a passenger vehicle. Without getting into to all the complexities and science behind an air conditioning system, I will narrow the failures down to the most common.

Blend Door Stuck in the Warm Position

Possibly the most obvious, is the blend door. The blend door is a key component for adjusting the temperature in the passenger compartment. It does exactly what the name suggests, it blends hot air from the heater core and cold air from the evaporator to create the desired temperature. The blend door is moved with an electric actuator motor. Blend door actuators fail more often in some vehicles than others. They can be difficult or easy to replace depending on where their located in the HVAC system. If the blend door is stuck in the hot position, the HVAC system will only blow hot air.

Refrigerant Leaks

Like your cooling system that contains antifreeze, your A/C system contains refrigerant. There is generally only one type of refrigerant used, and that would be R134a. There are two factors that contribute most to leaks in an A/C system. Number one would be the high pressures which range from 150-310 psi. The other property is R134a can be corrosive. Corrosion will occur if there is moisture in the system. This can occur from improper maintenance and or a lack of maintenance in an A/C system.

Your A/C system has many metal and rubber hoses connected to each other that connect to three main components. The condenser, evaporator and compressor. To create a sound seal between them, many rubber o-rings are used. Over time rubber dries out and begins to crack resulting in small leaks. As your A/C system ages, small leaks occur and eventually there will not be enough R134a in the system to create cold air.

Repairing leaks in the A/C system first requires a technician to locate the leak. This can be a difficult task at times due the small nature of the leaks. Often times an A/C system will have slow leaks that only become a problem in a matter of years. These small leaks are very difficult to locate so it is more cost effective to add refrigerant to the system. Larger leaks can be found by placing dye in the system or by using a special tool designed to detect escaping refrigerant. I’ve found the use of dye to be the most effective method, with the exception of listening for hissing, which would indicate a very large leak, and looking for wet spots on any of the A/C components. Wet spots are where oil in the system has leaked out and an obvious sign of leakage.

Once a leak is found, the A/C system should be evacuated and recovered for recycling. R134a does have negative effects on the atmosphere and should be handled with care. Having emptied the refrigerant from the system, you can now proceed to address the leak. Usually an A/C leak is a failed o-ring. It is not uncommon for the failure to be at the compressor. Not as common, but it still occurs, are leaks in the condenser from small objects on the road that have penetrated the condenser core.

The automotive industry has used several different kinds of refrigerants over the years. First in use was R-12 but its use was phased out by 1996 due to the negative effects it has on the ozone layer. R12’s replacement is R134a and is still in use today. Although, some newer cars come filled with R1234yf beginning in 2014, it is still not clear whether or not it will become the standard in the industry.

A/C Compressor Failure

The A/C compressor is arguably the heart of the A/C system. It relies on oil in the system for lubrication. If there are oil leaks that are not attended to, or a proper amount of oil is not added to the system when servicing, the compressor will have increased internal friction that will short its life. This condition usually results in the internal grinding of metal on metal surfaces resulting in a noisy compressor and eventual seizing.

If a noisy compressor is left unrepaired, eventually the metal shavings will dislodge and deposit in other parts of the A/C system. The result will be clogging somewhere in the system. Clogging in the system can lead to excessive high side pressures, or restrictions. Both prevent the A/C system from creating cold air. Flushing is required to repair this situation. It can be messy and time consuming depending on how complex the particular system is you are working on. A thorough flush is part of any compressor replacement.

Broken A/C Belt

This mainly occurs because of age. Belts are a regular maintenance item that if left undone, will result in the belt breaking. Without a belt, the compressor cannot operate and warm air from your vents is the result.

Belts are fairly foolproof outside of age. Other things that can result in their demise are frozen idler pulley bearings and debris from the road. Most belts are shielded from the road effectively, but bearings on the other hand fail with age as well. It’s common practice to replace a belt when replacing an idler pulley. If an idler pulley freezes and ceases to turn, the belt will be damaged and they will both need to be replaced.

Pressure and Temperature Switches (AKA Cycling Switch)

A/C switches come in multiple forms. To many to explain each one here. No matter their type, their function is the same. Their function is to make sure the low side does not freeze and the high side doesn’t burst. An A/C system left unchecked will do both. These switches either monitor pressure or temperature and cycle the compressor clutch accordingly.

Compressor Clutch

The compressor clutch is a electromagnetically controlled device that engages and disengages when voltage is applied to it. The clutch has what is known as an air gap that collapses creating contact between the clutch plate and compressor belt pulley. When these two come into contact, the compressor begins to spin compressing the R134a inside the A/C system. Some refer to the A/C compressor as a pump, which isn’t quite accurate, but not entirely wrong. It does have a pumping effect, but it’s primary mission is to compress the R134a gas contained inside. The nuances of this subject require some study of hydraulics which I won’t be covering here.

When the clutch plate and compressor belt pulley come into contact there is a great deal of friction created resulting in wear not unlike what your brake pads experience. As these two components wear, the air gap is increased and the ability of the electromagnet to pull them together is decreased. Eventually the clutch will begin to slip. The more slip that occurs, the more heat will build up resulting in the two components getting very hot and damaging the clutch beyond repair. Often times it damages the compressor as well. Through regular servicing of your A/C system this can be avoided.

When the clutch is slipping, the compressor is unable to compress the R134a refrigerant sufficiently and the system cannot operate at capacity. This symptom can often be confused as a low refrigerant condition. It takes experience to recognize that a slipping clutch is the problem. When diagnosing the A/C system, it’s important to be very observant of all the parts of the A/C system. Their operation are clues to what the problem is. An A/C system can be a complex thing to diagnose, but through thorough understanding of how an air conditioning functions, most diagnosis is fairly straightforward for seasoned technician.

As stated before, the compressor clutch has what is known as an air gap. This air gap can be adjusted restoring the clutch to its previous operating capacity. This process requires some specialized tools but is fairly simple to accomplish, providing the A/C compressor is easy to access. Many are not and will require the compressor to be removed which is a much larger and more involved job.

Condenser Fan Inoperative

The condenser is the second radiator mounted in front of your coolant radiator. But it isn’t really a radiator, it is a condenser. There is a cooling effect at play here, but that isn’t the primary task of the condenser. Confusing, I know. The condenser will have a fan mounted to it to enable the condenser to yes, cool the condenser.

What a condenser does is not unlike the condensation you see on your car windows in the morning. Condensation is when a gas turns into a liquid. In the case of your car windows, the gas is the air turning into water. In your A/C system the gas is the R134a being condensed into liquid R134a. This is part of the compression or high side of an A/C system. This process on your car window and your A/C system require a temperature change to occur. There is a direct correlation between temperature and pressure in our world. This phenomenon is a big part of the weather outside as well as the conditions in your A/C system. When working on an A/C system it is important to remember when temperature rises, so does pressure. When temperature falls, so does pressure.

When condenser fans fail, the condenser temperature will rise beyond it’s ability to condense the R134a inside. A condenser fan failure will often only be noticed when stopped in traffic because as a car moves at freeway speeds, air is pushed through the condenser with a greater effect than even the condenser fan can provide. The condenser fan is only needed during low speeds where there isn’t enough air moving through it.


The condenser, as stated above, is mounted on the very front of your vehicle just behind the front bumper. It is very vulnerable to punctures from road debris. When this occurs, there will be a loss of R134a resulting in an inoperative A/C system.

Condensers are also vulnerable to internal clogging since it is the first component after the A/C compressor. Compressors require oil to alleviate high friction inside them. When things begin to fail inside an air compressor, the metal shavings will be lodged in the condenser creating a clog. When this occurs, the A/C system should be thoroughly flushed along with replacing the compressor.

Power Switch and Fuses

Aside from the major components mentioned above, it’s not uncommon for the A/C switch on your dash to fail. If the switch fails, the system will not receive a signal to turn on. Working closely with the switch would be the fuses that provide power to the various parts of an A/C circuit. Fuses sometimes fail due to moisture and corrosion, but they usually fail because another part of the system has a problem. Fuses are used to insure high electric currents don’t occur which can cause fires. Simply put, fuses are simple safety devices.

If you need any help with the diagnoses of a problem with your A/C system, feel free to contact a professional technician who can pinpoint any issues and repair your air conditioning system accordingly.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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