Why Does My Car AC Stop Working When Hot Outside? Answered

Sometimes, you might notice that your car’s AC randomly turns off and on. At other times it just blows hot air rather than cold air, especially when it is hot outside.

When this happens, it can be pretty uncomfortable and inconvenient, but this indicates that your car has developed an issue and needs maintenance or a service repair.

Here are the main causes why your AC stops working when hot outside

The cause of this issue might be any of the different components of the AC system. However, most times, the main cause stems from pressure changes. The moment the AC system cannot keep up with the pressure changes, the AC doesn’t work appropriately.

Other causes include:

  • Low on freon
  • Broken AC compressor
  • Faulty ventilation fan
  • Faulty expansion valve
  • Faulty evaporator temperature sensor

Low on freon

The air conditioning unit of your car relies on pressure differences. The freon (refrigerant) starts in a gas state at low pressure, then is changed to liquid at high pressure. Without appropriate freon levels, the AC unit does not circulate cold air anymore and might even start releasing warm air instead. Low freon levels could be caused by leaks in the pressurized compartments over time.

Common signs of low freon levels

  • AC is circulating warm air
  • Noticeable refrigerant leaks
  • AC clutch refuses to engage

If a refrigerant leak occurs, the AC unit becomes incapable of functioning normally. Once you observe a leak, it should be attended to immediately as it could cause health hazards and increase energy consumption. You generally do not have to replace your AC refrigerant if a leak is not present. In case of a leak, a professional should be consulted.

How to replace refrigerant in car’s AC

You can replace refrigerant in your car’s AC unit as long as it uses r134a refrigerant.

Before replacing the refrigerant, ensure that you do not have any leaks present. You can check for leaks by spraying dish soap and water on the different components of the unit. If you notice bubbles on any part, you might have a leak. Lots of foaming will indicate a large leak. For excessive leaks, you might need to hire an expert. Small leaks can be repaired with an AC recharge kit that contains a sealant.

After confirming the absence of leaks, you can continue the recharge. Check the unit’s pressure using a pressure gauge on the low refrigerant line labeled ‘L.’ The high refrigerant should not be refilled with refrigerant.

To avoid overcharging, use a recharging kit with a pressure gauge. Before handling refrigerant, ensure that you wear appropriate protective equipment as the substance is dangerous to health.

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The compressor is broken

The compressor is the unit that compresses the refrigerant before transferring it to the condenser. Then at the condenser, the refrigerant changes state, gas to liquid. Consequently, a bad compressor will result in a faulty or broken AC unit.

Common signs of a broken compressor

If you can spot these signs early on, you might be able to replace your air compressor before it causes more damage and increases repair costs.

The following are common signs that point toward a faulty compressor,

  • Bad suction lines
  • High cabin temperature
  • Fluid leak
  • Noise emanating from the compressor
  • Tripping circuit breaker
  • Stiff compressor clutch

How to replace an AC compressor

Before attempting to replace the compressor, ensure you are not replacing a fully functional unit. Replacing an AC compressor is quite pricey, with the average cost ranging from $200 to $1000 depending on the model of the car.

Replacement at a shop might cost more due to additional labor costs. The replacement should be left in the hands of an expert, but if you’re efficient with tools, you might be able to get it done with guidance.

The first step is to remove the compressor from the engine. Before attempting to remove it, ensure you disconnect the car’s battery to prevent short circuits or electric jolts. Try to locate the serpentine belts.

You might consult a service manual to help you locate the parts of your engine you need to access. Once you find the belt, loosen the belt tensioner. Then try to locate every tube on the compressor. All connections need to be disconnected before you attempt to remove the compressor.

Depending on the model of your car, you might have to loosen other attachments before getting to the compressor. As you loosen the last bolt of the compressor, ensure you hold on tight to the compressor so as not to cause any hazards.

Just to be sure, after you remove the compressor, inspect it to figure out why it failed or if it is even the actual cause of the failing AC unit.

Before installing the new compressor, compare it to the old compressor for easy installation. The designs might be slightly different, but the accessories should be similar. Confirm from the compressor’s user guide if the compressor has pre-added oil. If it doesn’t, pour in the specified amount of oil.

Before mounting the new compressor, you might consider replacing the old o rings to prevent future refrigerant leaks.

Then you can mount the new compressor unto the space of the previous and reconnect all disconnected parts such as tubes and serpentine belt.

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The ventilation fan needs to be replaced

Ventilation fans blow cold air through the vents. If the ventilation fans are bad, the AC unit might not blow air through the vent. However, if the fan blows warm air, the issue might be from the refrigerant, condenser, or compressor.

Common signs of bad ventilation fan

  • Low airflow
  • Strange smells from the vent
  • Noises from the vent

Replacement of AC fan in a car

To replace the ventilation fan motor of your car’s AC unit could cost about $200 to $800, but if your AC fan motor is under warranty, it could be as less as $100 to $300. Self-replacement might cost below $100. The ventilation fan motor can easily be accessed through the glove box.

A service manual can be used as a guide to locate the ventilation fan. After unscrewing and loosening the former, the new one can easily be installed. The process is pretty straightforward.

Faulty expansion valve

The evaporator valve eliminates pressure from the refrigerant when in the liquid state to allow a change of state into gas in the evaporator.

Common signs of a faulty expansion valve

  • Warm air circulation
  • The compressor is constantly on
  • Irregular airflow
  • Presence of frost on vent
  • Failing AC unit

Replacement of an expansion valve

It costs about $200 to $500 to replace your AC’s expansion valve at a workshop, including labor costs. However, replacing it yourself will cost about $100 to $200.

  1. To replace the expansion valve, you will need to empty the system. the refrigerant needs to be recovered via the low-pressure line of the AC system. The recovery might take a while if the system is full- about 20 minutes.
  2. Then locate the expansion valve and remove it. When installing the new expansion valve, replace the O rings as well to prevent refrigerant leaks.

Faulty evaporator temperature sensor

The evaporator temperature sensor observes the temperature of the evaporator.

Common signs of faulty evaporator temperature sensor

  • The temperature of the AC fluctuates
  • Warm air from the vent
  • Inactivation of compressor

Replacement of faulty evaporator temperature sensor

The replacement cost is about $300 to $400, including labor costs, but replacing the sensor yourself could reduce the cost to below $100.

  1. Disconnect the negative terminal of the battery
  2. Locate the evaporator temperature sensor
  3. Remove the sensor
  4. Install the new sensor.

Is it safe to drive my car if the AC stops working?

Although I recommend identifying and fixing any issue that may arise as soon as possible, driving a car with a bad AC will not put you at any risk. You will only experience the discomfort of driving in hot weather without AC.

This can usually be solved by rolling down the windows to allow some air ventilation inside the car.

In summary

This article described the main causes for which your car might start blowing hot air in the summer and how one would go about fixing each one of them. However, if you feel like you lack the mechanical skills needed to diagnose and repair the car yourself, you should look for a repair garage in your area.

As usual, I recommend checking some online reviews before driving to the place to prevent any unnecessary bills and ensure your car is being fixed by a professional. If you have been a ca owner for some years, you probably stumbled upon all sorts of mechanics, some less skilled than others.