Car AC Not Working? Weak airflow? Top 8 Things to Check

When your car's air conditioner isn't cooling anymore — or worse, blowing hot air — it can make for an uncomfortable ride on a warm day. The solution could be a simple one, but it could also require a complex repair that would be risky to attempt at home.

There are two basic types of problems you can experience with your car's air conditioning: the AC is blowing hot or warm air, or the vents have little to no airflow. Each one will have different symptoms and different potential solutions. Choose your issue below to see more details.

Most common reasons the AC is blowing warm or hot air

• AC refrigerant leak
• Climate control electrical issue
• AC compressor problem
• AC condenser fan not turning on
• Heating System Issues

Most common reasons for little to no AC airflow

• Cabin Air Filter
• Blower Motor
• Vent Control System

Keep reading to find out how these components work and how to diagnose them.

Car air vents / Image source

Most common reasons the AC is blowing warm or hot air

While there are a variety of reasons your car's air conditioning might not be operating at top performance, the most common are a refrigerant leak, an electrical climate control issue, a problem with the air conditioning compressor, a problem with the condenser fan, or an issue with the heating system. Each of these can cause the AC to blow lukewarm or hot instead of cold air.

1. AC refrigerant leak

Your car's air conditioning uses refrigerant, a fluid designed to vaporize at a low temperature, to cool the air before it blows through the vents into the passenger compartment of the vehicle. If that refrigerant is leaking from somewhere in the AC system it will become less effective at cooling the air before it leaves the vents.

A leak like this can be the result of a faulty component within the system, such as the compressor, evaporator or hoses. Or, it can just be a sign of age for an older vehicle, since the parts and hoses use rubber seals that naturally tend to wear over time, causing the refrigerant to leak.

Accidents and other damage can also cause refrigerant leaks. Since the AC condenser is located in front of the radiator, it's susceptible to damage from flying road debris and other small front-end impacts. Another possibility is that AC hoses and pipes can rub against other parts while you drive, eventually causing a leak.

Air conditioning system

Diagnosing an AC refrigerant leak

One way you can verify this is by listening to the AC compressor. You should hear an audible click from the engine area when the AC is turned on. This is the compressor clutch engaging, which allows the compressor to operate.

When a system is low on refrigerant, the compressor will cycle on and off frequently. If the system is very low, the compressor will not turn on at all. The system must be checked for leaks; any leak found should be repaired and the system properly evacuated and recharged.

How to find a leak (when the engine is off):

• Listen for a hissing sound from the AC system
• Look for oil residue from or around AC hoses and pipe fittings
• Inspect the condenser for road damage (in front of the radiator)
• Use a special tool that can detect AC refrigerant gas
• Use an infrared light to detect ultraviolet dye in the refrigerant (if previously added)

Solution: If a specific part of the AC system is the culprit, the repair will involve an inspection and repairing or replacing the offending part. If enough refrigerant fluid has already been lost, a full AC recharge will also likely be necessary in order to flush the system and replace it with the right amount of refrigerant to function properly and keep air blowing cold.

Note: Refrigerant is a dangerous and toxic chemical, and should be kept from escaping into the atmosphere. Because of this risk, only trained technicians with the proper equipment should perform air conditioning repairs.

2. Climate control electrical issue

The buttons, knobs and control screens that you use to adjust the temperature in your vehicle tell an AC/heater control module — basically a computer — to pass instructions to the heating and cooling system.

Just like with any electrical tool or appliance, this component or the parts that support it, such as electrical wiring, can break down or fail for a variety of reasons. As a result, your car's AC could start to perform inconsistently, or even stop working altogether.

Solution: Due to the complex nature of the electronics in this system, an experienced and trained technician will need to evaluate the components to diagnose the true root cause, and ultimately repair or replace the faulty parts. You can help by clearly communicating and demonstrating the symptoms with the shop before they start working to fix this issue.

3. AC compressor problem

The compressor is an extremely important part of your car's air conditioning system. Its purpose is to move refrigerant through the system, and compress low-pressure refrigerant gas into high-pressure refrigerant gas before passing it to the AC condenser.

If this crucial component starts to break down, it will mean refrigerant can no longer be reliably moved through the system, so air blowing through the vents can't be cooled.

AC compressor

Diagnosing an AC compressor problem

The compressor must turn on for the AC system to work. The compressor belt and the electrical circuit to the compressor must be in good condition. If the belt is OK, the mechanic will check for proper power supply to the compressor. If that checks out, then the compressor has likely failed and may need replacement.

When the AC compressor is replaced, you may need a few additional items:

• Replace the AC accumulator or receiver dryer (this part helps to remove moisture from the refrigerant)
• If the compressor failed internally, then metal shavings may have been distributed throughout the system. This would require a system flush and potentially the replacement of other damaged parts.
• When replacing the AC compressor, the drive belt should be replaced if it shows any sign of wear

If the power supply is not present, the problem might be:

• Defective wiring or fuse
• Bad pressure switch
• Bad control module
• AC operation criteria has not been met:
  1. Low refrigerant pressure (low refrigerant or low pressure switch)
  2. Outside temperature too low (ambient temperature sensor)
  3. High refrigerant pressure (engine temperature too high/blockage in refrigerant system)
  4. Engine at full throttle (throttle position sensor)

Solution: An inoperative compressor is normally due to low refrigerant or loss of power to the compressor. However another solution may involve an AC compressor replacement.

4. AC condenser fan not turning on

Most cars have an electric cooling fan in the engine bay that starts spinning when the AC is turned on. Some cars may have multiple fans. If the AC condenser never spins, it could be the reason that your AC isn't as cold as it should be.

Diagnosing an AC condenser fan that doesn't turn on Fans and electronics can be dangerous - be safe when doing any of the following tests. Open the hood and locate the AC condenser fan. Your car may have multiple electric fans, identify which one is the AC condenser fan by consulting the user manual. Some manufacturers may call this fan an "auxiliary fan". Start the vehicle, let it idle until it reaches operating temperature. Turn on the AC. While the AC is on, the condenser fan should start spinning. If it doesn't spin, check the condenser fan fuse and relay and replace if necessary.

If it still doesn't spin, the problem might be: - Defective wiring - Bad fan switch, relay not getting signal to send power to the fan - Fan itself is bad

Solution: Fans and electronics are dangerous on their own, and even more dangerous together. Have a certified technician take a look at your vehicle. Depending on the issue, a fuse or relay may need to be replaced, wiring may need repair, a switch may need to changed, or the fan itself may need to be replaced. Like all electrical systems in a vehicle, the operation of an electrical fan can have many failure points.

5. Heating System Issues

In some vehicles, airflow to the vents is routed through the both the heater core and the AC system. The heater is controlled by a heater control valve that diverts hot coolant from the engine. If the valve is not functioning correctly, it can be sending hot coolant through the heater core at all times. Cold air from the AC will pass through the hot heater core and result in warm air in the cabin. So the AC might be functioning correctly, but the heater is still "on".

Diagnosing a malfunctioning heating system

By hot air, we mean air that is even hotter than outside and similar in temperature to having the heater turned on. Turn off the AC, but set the air temperature to the coldest level. If hot air is still coming through the vents, it is possible that the heating system is malfunctioning.

Common reasons for little or no AC airflow

Air flow is created by a "blower motor" and directed through a cabin air filter and passageways in the dashboard using flaps and doors. The blower motor and the passageway flaps and doors can fail causing airflow issues. It is best to check the cabin air filter first, because of how easy and cheap it is to replace it. Here are common reasons why there is little or no airflow from the vents:

1. Cabin Air Filter

The most common reason for little or no airflow through your vents at any fan speed is the cabin air filter. Cabin air filters clean the air coming into your car through the heater and AC. If your cabin air filter is a few years old, or if you've never changed this filter, it can be filled with tons of dust and debris from the outside world. Old filters can have dirt, feathers, sticks, rocks, and tons of nasty debris that is blocking all the air from going into your heater or AC system.

Diagnosing a a bad cabin air filter

Finding out if your cabin air filter is pretty simple. Typically, cabin air filters are very accessible. Depending on your make and model, the cabin air filter can be under a plastic cover on the outside of the car at the base of windshield, under the dashboard, or under the glovebox. Take the filter out and inspect both sides.

It is not recommended to turn the fans on with no cabin air filter installed. Debris and dust can make its way deep into the air passageways where it is difficult to clean, and could be a source of mold growth.

Solution: If either side of the cabin air filter is dirty, or if there is a lot of dust or dirt in the housing of the filter, use a vacuum cleaner to remove all the debris in the housing and install a new air filter. Thankfully, this one of the easiest and cheapest maintenance items you can take care of on your car. If your filter was really dirty, you should notice the airflow improve significantly.

2. Blower motor

If your cabin air filter is in good condition and you're still not getting good airflow, you can check the blower motor next. The blower motor is an electronically controlled fan that blows air into the passenger compartment. The motor can stop working for several reasons, all which cause the flow of cool air to cease. There are two main reasons this occurs: the motor or the system controlling it have failed.

Diagnosing a bad blower motor Make sure your cabin air filter is in good condition. Change the fan speed to maximum. If your hear a grinding, squeaking, or rubbing noise that gets louder as you increase the fan speed, the blower motor likely needs to be replaced. Change the airflow direction (upper vents, lower vents, and both). If airflow is weak in all vents, then your blower motor may need to be replaced. Also, check with the AC on and off and with different temperature settings. A bad blower motor will most likely have weak airflow no matter what temperature settings you choose.

Solution: On most vehicles, a bad blower motor will need to be replaced entirely. On some vehicles, you may not need to replace the entire motor. Some types of blower motors can be serviced with new brushes or fan cages.

3. Vent Control System

Depending on your make and model, the flaps that control airflow through the different vents in your car may use different kinds of operations to control their position. A vent can become stuck closed or stuck open. This can happen because of something simple like dust and debris build up, the flap itself broke or snapped, or if the system that controls that flap has an issue. There are different kinds of systems that control vents:

Cable operated vents

These control systems are quite simple. You have a lever or knob, a cable, and a cable-controlled door or flap. When you move the lever or knob, the cable opens and closes the door, changing the direction of the airflow. The problem is usually broken cables, but the controls can break, as well.

Vacuum operated vents

These systems use vacuum to operate a vacuum motor (or diaphragm), which in turn operates a door or flap. The controls can be mechanical or electronic, but the system will use vacuum hoses to transmit vacuum to the vacuum diaphragms. Most of the time, the problem is a vacuum leak.

Typically, when these systems lose vacuum, the airflow is automatically directed to the windshield — this may be constant or it can occur just when you're driving. If the airflow stops when you're accelerating, this is likely due to a vacuum leak.

Electrically operated vents

Most newer vehicles have an electrically controlled system, which uses electric motors to change the door position. A control module will send a voltage signal to the appropriate electric motor, which opens and closes the door accordingly. Some vehicles can have problems with the motors, while others have control module issues. Whatever the cause, these systems take electrical diagnostic skills to be able to diagnose and repair.

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