Why Is My Car AC Blowing Hot Air? 12 Reasons

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There are several reasons behind a car AC blowing hot air, such as issues with the compressor, condenser, blend door actuator, air filters, Freon levels, or electrical components. Ensuring your in-cabin features are functional is very important, especially during summer. Otherwise, you may have to brace yourself for profuse sweating and uncomfortable rides.

Here are 12 reasons why a car AC blows hot air:

  1. Freon leak
  2. Broken condenser
  3. Flawed compressor
  4. Dirty evaporator coils
  5. Restricted air filters
  6. Failing blend door actuator
  7. Bad AC pressure switch
  8. Faulty evaporator temp sensor
  9. Microorganism buildup
  10. Blown AC fuse and other electrical issues
  11. Overcharged refrigerant
  12. Contamination

Although non-exhaustive, this list aptly covers the most prevalent causes behind AC malfunction. Additionally, this article aims to equip you with the right tools and knowledge to best address this issue.

Some of the steps detailed here can be done even by nouveau car owners. More complex root causes would require the assistance of a licensed mechanic or technician.

Why Is My AC Blowing Hot Air in My Car?

Car Air Conditioner

1. Freon Leak

Inasmuch as insufficient coolant affects your vehicle’s heating system, the same can be said for its cooling system if the refrigerant requirement is unmet. Typically, a brand-new sedan holds between 28 and 32 ounces of refrigerant, while vehicles with rear A/C likely hold more (refer to your owner’s manual for specifics). But over time, this pressurized system weakens and either develops leaks or drops its pressure levels.

A refrigerant leak predominantly leads to the AC blowing hot air in your car. This problem mostly happens after your vehicle meets an unforeseen accident but can also be caused by a leaking condenser or evaporator core, cracked hose, or leaking O-rings.

Damage to the AC system joints (the spot where hose leaks usually trace back to) is not that easy to pinpoint — even for experienced vehicle owners. Should you suspect a refrigerant leak, the best places to look for that thin, greasy substance would be under the hood (around the compressor), inside the cabin, or below your vehicle.

2. Broken Condenser

The condenser is one of your vehicle’s A/C system’s five components and helps keep the cabin air cool. It is behind your car’s front grille (in front of the radiator). It utilizes ambient air passing through it to remove heat from the circulating refrigerant. Subsequently, the condenser converts the same into a high-pressure liquid refrigerant that goes back into the system.

Apart from hot air coming from the A/C, symptoms of a bad condenser include noticeable leaks beyond the typical amount of condensation and loud or unusual noises from the AC unit. You may also notice that your AC vents blow hot air when driving slowly or during extended idling (when caught in traffic). If you encounter any of these, have your vehicle serviced immediately. Although keeping the condenser free of debris is still the best way to ensure its continued functionality.

3. Flawed Compressor

Possibly the most important of the five air conditioning components is the AC compressor. It is the part responsible for circulating refrigerant in the AC system and for passing the same to the condenser. Expect hot airflow inside your vehicle if the compressor turns bad or fails, as there is nothing to circulate refrigerant throughout the AC system.

When examining this area, I highly advise inspecting your AC compressor belt and clutch. Although quite simple, it is vital to the proper function of your AC compressor as it connects the latter to the power mill. This belt enables the AC compressor to be turned with the engine’s power when your car is running. Without it, your AC compressor will not be able to pressurize your vehicle’s air conditioning system.

While at it, closely examine your expansion valve and orifice tube too. These components risk clogging when an AC compressor fails due to metal shavings that may come from the latter. Depending on the compressor’s damage, it may warrant a replacement. If so, flush out the contaminants from your air conditioning system before installing the new part.

4. Dirty Evaporator Coils

Evaporator coils are where hot air is absorbed and cooled by refrigeration and is part of the evaporator — another component of the A/C system. However, its ability to effectively cool the air inside the cabin decreases if it is clogged or dirty.

Your cabin air filter augments the evaporator coils’ capability by capturing as much dirt and airborne particles as possible. However, it cannot filter at 100% since moisture and humidity make it conducive for pollutants to build up on the fins, blocking airflow through the evaporator.

Fortunately, it is easy to clean the evaporator coils. All you need is a screwdriver, a catch pan, a spray bottle filled with water, a hairdryer, and an AC evaporator cleaning product like Nu-Calgon 4171-75 Evap Foam No Rinse Evaporator Coil Cleaner (view on Amazon). When ready, follow these steps:

  • Place the catch pan under the evaporator discharge line (located under the vehicle).
  • Remove the glove box.
  • Remove the resistor block or blower fan assembly to access the evaporator coil. If these two prove futile, drill a 3/8″ hole between the blower fan and the evaporator case (only do this latter option as a last resort).
  • Spray the foam-based evaporator cleaner onto the evaporator coils (though the access point).
  • Wait 30 minutes, then rinse using your water spray bottle.
  • Dry the evaporator coils using a hairdryer.
  • Put the resistor block or blower fan assembly and glove box back in place.

5. Restricted Air Filters

Dirty Car Air Filter

A common culprit behind an AC blowing hot air (as opposed to its contribution to dysfunctional car heaters) is a dirty air filter. The reason is that dirty air filters create airflow problems and push your air conditioning to work twice as hard, increasing the propensity of a blown fuse due to an overstressed electrical system.

6. Failing Blend Door Actuator

Like air filters, the AC actuator (or blend door actuator in Jeeps) affects the heating and cooling processes of your vehicle’s air conditioning system. As established in a previous article, this part controls the air temperature blowing through the A/C vents and into the cabin.

While it can deteriorate due to normal wear and tear, the AC actuator can fail due to faulty electrical connections or a burnt-out actuator motor. Another cause is flimsy actuator gears, usually made of plastic. Slipping or grinding gears are often accompanied by droning, clicking, or squeaking noises from the dashboard when you turn on the ignition.

An actuator recalibration may be enough to rectify this issue if only a faulty relay is suspected as the culprit. To ascertain this, first, do the recalibration before checking on the status of the relay. If either or both procedures eliminate the clicking noise, then the problem is resolved. If not, you might need to replace the entire blend door actuator unit — that is, after ensuring all electrical issues are fully addressed.

7. Bad AC Pressure Switch

This sensor detects and monitors refrigerant pressure in your vehicle’s air conditioning system and is a crucial piece of the puzzle. If this small component becomes defective, it will send incorrect values to the climate control unit, causing the AC compressor not to turn on. When this happens, you would automatically get hot air blowing from your car’s AC vents.

A high-spec OBD-II or DRB-III scanner like Launch X431 V+ Elite PRO 4.0 Key Programmer, ECU Coding & Bi-directional Scan Tool (view on Amazon) should help diagnose the suspected pressure switch. Getting any Diagnostic Trouble Code from P0530 to P0533 or getting pressure readings of zero from the sensor would mean an actual fault with this component. Before jumping into replacing more expensive parts like the compressor, make sure to check first if this little guy is working fine.

8. Faulty Evaporator Temp Sensor

The evaporator temperature sensor has similar functions as the AC pressure switch, except that it does so for the evaporator coil in the AC system. The vehicle’s control unit turns the AC compressor on or off based on data received from this sensor. Hot air blowing from your AC vents should be the least of your worries, as this little fellow can also cause an evaporator freeze-up if it develops a fault.

9. Microorganism Buildup

One of the reasons your car AC may be blowing hot air is due to clogged AC vents. This occurs when you keep the air conditioning on for long periods, resulting in microorganisms propagating inside your cabin, blocking the AC vents, and adversely affecting the HVAC’s cooling process.

Taking your vehicle for servicing at least twice a year is a great remedy to this problem. Doing so clears your car’s AC duct and eliminates in-cabin dust particles and bacteria, keeping your daily driver hygienic and pleasant to drive in. Moreover, this practice adds to the longevity of your car’s A/C system.

10. Blown AC Fuse and Other Electrical Issues

Unwanted short circuits in the cooling coils or any other part of the electrical unit running your air conditioning is another factor that can cause the car AC to blow hot air or stop working altogether. And the climate has a lot to do with it. If you are in a location where the ambient temperature tends to exceed 45° C, your electrical wires could likely get toast and result in a short circuit in the internal wiring.

It may be possible to diagnose it and do a quick DIY repair — like sealing the area where the wires have been shot with electricity-resistant tape. But if your car insurance covers this dilemma, it is best just to sit back and outsource the task to an authorized car repair shop.

11. Overcharged Refrigerant

Checking Car Air Conditioning System Refrigerant Recharge

An overcharged refrigerant has the same effect as a refrigerant leak on your vehicle’s A/C system and is another reason your car AC would be blowing hot air even after recharge. It not only adversely affects your HVAC system’s cooling performance but can also lead to a major leak or damaged AC compressor (view on Amazon).

A well-functioning A/C system with an adequately charged refrigerant should blow air approximately 40° to 50° colder than ambient temperatures (this is only a ballpark figure). OEM-recommended values should allow for changes in atmospheric temperatures and their corresponding effect on refrigerant pressure. But if you overcharge the refrigerant, you disrupt the balance between the two and cause your AC system to overpressurize when ambient temperatures get warmer.

12. Contamination

Refrigerant or air contamination is often overlooked. Because refrigerants are stored in pressurized, sealed containers, very few people question their purity and, more so, if they have anything to do with a malfunctioning AC unit. Unfortunately, this lack of alertness has, in many ways, aggravated A/C system problems.

It is worth noting that air is a non-condensible gas — meaning it cannot change from vapor into liquid form at the same pressure levels an AC compressor can achieve. Because of its nature, air ends up occupying space and displacing refrigerant inside your AC unit, resulting in the following complications:

  • Clogged expansion valve or orifice tube
  • Compressor failure
  • Evaporator freeze-up
  • Increased compressor noise
  • Outlier discharge pressures or high-side readings
  • Sporadic or reduced cooling performance

Other Problems

Ultimately, air trapped inside the A/C system causes contamination. But there are many facets to this scenario. Elements like dirt and outside air or refrigerant cross-contamination from improper servicing are a few other ways air gets inside this closed, pressurized system. Adding refrigerant to your vehicle at home or even a charging station is also to blame.

Some charging stations have recycling machines that automatically vent trapped air from the recovery tank. However, others continue to use equipment that requires the process to be manually done — and herein lies the problem. The equipment’s age is also a concern, as dated machines do not automatically purge recovery tanks as well as they should.

It also does not help that we are currently in a multi-refrigerant market. Vehicles that use R-12 are still around. Newer models are split between R-134a and R1234yf variants. Many auto parts stores offer alternative refrigerants, such as Freeze 12, FRIGC, Free Zone, and R-406A. And mind you, the purity of these variants is subject to the diligence of shop technicians in checking the pressure of refrigerant recovery and storage tanks and ensuring it does not go beyond spec.

Conclusion – Why Is My Car AC Blowing Hot Air?

To recap, here are 12 common causes of a malfunctioning AC unit:

  1. Freon leak
  2. Broken condenser
  3. Flawed compressor
  4. Dirty evaporator coils
  5. Restricted air filters
  6. Failing blend door actuator
  7. Bad AC pressure switch
  8. Faulty evaporator temp sensor
  9. Microorganism buildup
  10. Blown AC fuse and other electrical issues
  11. Overcharged refrigerant
  12. Contamination

Even without these challenges, the average lifespan of a car AC is 15 to 20 years — sometimes just a little over a decade. However, this short-lived timeframe should come as no surprise.

Like everything else mechanical, the components of your vehicle’s air conditioning system are bound to fail over time. This natural deterioration is why stringent adherence to routine maintenance is utterly significant, as these procedures add to the longevity and performance of your AC unit. If you have followed most (or all) of the steps in this guide and continue to get hot air blown out of your AC vents, it may be time to seek professional help or look into replacement options.

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