How to Diagnose a Non Working Air Conditioning in a Car

Co-authored by Tom Eisenberg

Last Updated: December 8, 2022 References Approved

Driving around on a hot day in a car without a working air conditioner can be uncomfortable and even dangerous in extreme heat. Diagnosing the reason that your air conditioning isn’t working will help you determine whether the problem is one that you can fix yourself or if you need to take it to a repair shop. You are also less likely to be taken advantage of at a repair shop if you already know why your AC is not working.

Method 1

Method 1 of 3:

Collecting Initial Information

1
Turn the air conditioning on while the car is running. The air conditioner won’t run properly unless your engine is running. The best setting for diagnosis is “fresh air” (not recirculated) with the air blowing out of the center vents in the dash and the AC turned on.
- Start with the fan speed turned to its highest setting.
- If your car has a “Max AC” setting, select that option.
2
Listen for unusual noises coming from the AC. Noise could indicate that there is a problem with your compressor and that it needs repair or replacing.^{[1]
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Expert Source

Tom Eisenberg
Auto Technician
Expert Interview. 26 July 2019.}

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3
Feel the air coming out of the vents. You will need to know if the air is cold, room temperature, or hotter than the air around it. Also note if it starts cold but turns warmer, or if it is usually warm but blows cold intermittently.
4
Notice the air pressure. Turn the air pressure to the high and low settings and see if the airflow changes like it normally does.
5
Smell the air coming from the vents. If there are unusual smells, you could have a leak. You may also need to change your cabin air filter.^{[2]
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Expert Source

Tom Eisenberg
Auto Technician
Expert Interview. 26 July 2019.}
- The location of your vehicle’s air filter will differ from brand to brand, so refer to your user’s manual to find the exact location.^{[3]
  X
  Expert Source
  
  Tom Eisenberg
  Auto Technician
  Expert Interview. 26 July 2019.}
- You should swap your AC filter out every 15,000 miles (24,000 km) or so.^{[4]
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  Expert Source
  
  Tom Eisenberg
  Auto Technician
  Expert Interview. 26 July 2019.}
6
Inspect your car’s fuses. Check your owner’s manual for the location of your car’s fuse panel, as it may be under the hood, in the trunk, or even in the driver’s foot well area. A blown fuse can cause your AC to stop working.^{[5]
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Expert Source

Ed Beery
Automotive Specialist
Expert Interview. 19 November 2021.}

Method 2

Method 2 of 3:

Diagnosing Airflow Problems

1
Check all of the vents. Be sure that air pressure is coming out of the vents that you have selected. Move the vent selector to see if the air moves to the proper vents.^{[6]
X
Expert Source

Tom Eisenberg
Auto Technician
Expert Interview. 26 July 2019.}
- If changing the selected vent does not alter the flow of air, you likely have a blend door problem, which will require replacing the doors inside the dash that determine the direction of the air flow.^{[7]
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- Blend doors change position when the temperature selection is changed, blocking the flow or permitting the flow of either hot or cold air.
- Sometimes an AC system with a mode door problem could be working fine, but the air flow is being directed somewhere else, like back out to the engine, instead of into the car.
2
Look at your cabin air filter. Especially if the air coming from your vents smells funny or if you think you have had a slow decrease in pressure for some time, check the air filter. You will be able to see if it has a buildup of dirt or debris on it.
- It is possible that your cabin air filter is so blocked that it is interfering with your air pressure, and replacing it will be a relatively easy an inexpensive fix to your problem.
- Your car manual may have instructions on replacing the cabin filter. If it doesn’t, try searching online for “replace cabin air filter” followed by the year, make, and model of your car (for example, you might search for “replace cabin air filter for 2006 Toyota Camry”).^{[9]
  X
  Expert Source
  
  Tom Eisenberg
  Auto Technician
  Expert Interview. 26 July 2019.}
3
Check for a blower motor problem. The easiest way to do this is to try turning on your heat. If you have weak air flow when the heat is on as well, your blower motor may have gone out.^{[10]
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- A blower motor may have a resistor problem if your air only blows on the high setting but will not blow on a lower setting.
- Unfortunately, mice and other rodents sometimes build homes in car HVAC hoses and can get caught in the blower motor when the car is started. A loud noise (or bad smell) coming when the heat or air are running could be a sign of this problem.

Method 3

Method 3 of 3:

Diagnosing Air Temperature Problems

1
Find the front of your AC condenser. It is normally located in front of your radiator. If there are leaves or other dirt or buildup blocking it, remove them and clean the area.
2
Look under the hood at the AC compressor clutch. If your air pressure is normal but the air is hot, you may have a compressor problem. Checking to see if the compressor clutch is engaging is a simple visual inspection.^{[11]
X
Expert Source

Ed Beery
Automotive Specialist
Expert Interview. 19 November 2021.} The compressor is usually located towards the front of your engine, just inside the grill of your car.
- Your car should be running with the AC on to check the compressor clutch.
- The compressor looks like a small motor with a large wheel on the end. The wheel (which is the compressor clutch) should be spinning. If it is not spinning, you have a problem with your compressor.^{[12]
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3
Check the tension on the belt of the compressor. It should be tight. If it is loose, you need a new compressor belt.^{[13]
X
Expert Source

Ed Beery
Automotive Specialist
Expert Interview. 19 November 2021.}
4
Find any coolant system leaks. One of the most common AC temperature problems is low refrigerant. The AC system is closed, so refrigerant should not be lost unless there is a leak.^{[14]
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- Look for oily residue on or around hoses that attach AC components together. Oily spots may indicate a refrigerant leak.
- You may consider using an electronic leak detector, which can detect much smaller quantities of refrigerant.
- There are also some testers that use dye, UV light, and protective glasses to find leaks.
- If you do find a leak, you will likely need to hire a professional to repair the leak. You will probably need new parts too, as many of the components cannot be repaired or patched.
5
Check for freezing. If your AC blows cold initially but then stops being cold after some use, it could be freezing. Excess air and moisture in the system may lead components to freeze (literally).^{[15]
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- Freezing can also be caused by an oversaturated receiver/drier or accumulator.
- Turning the system off for a while and letting it thaw will temporarily solve the problem.
- If the problem is persistent, you may need your system to be flushed or evacuated with a vacuum pump.

Community Q&A

Question

Can a bad temp sensor cause AC not to work?

Ed Beery
Automotive Specialist
Ed Beery is an Automotive Specialist and the Owner of InTechgrity Automotive Excellence based in Denver, Colorado. With more than eight years of experience, he specializes in providing maintenance and repair services for both individuals and companies. Ed and the InTechgrity Automotive Excellence Team are approved by the American Automobile Association (AAA) for repairs and are Automotive Service Excellence (ASE) certified.

Ed Beery

Automotive Specialist

Expert Answer

Definitely! If the temperature sensor does not send a signal, then the AC will not turn on. Just keep in mind that these sensors are located deep in your HVAC system behind the dashboard, and can be difficult to inspect and replace.
Question

How do you test a blend door?

Ed Beery
Automotive Specialist
Ed Beery is an Automotive Specialist and the Owner of InTechgrity Automotive Excellence based in Denver, Colorado. With more than eight years of experience, he specializes in providing maintenance and repair services for both individuals and companies. Ed and the InTechgrity Automotive Excellence Team are approved by the American Automobile Association (AAA) for repairs and are Automotive Service Excellence (ASE) certified.

Ed Beery

Automotive Specialist

Expert Answer

Start your vehicle and turn on the AC with the fan on high. Then, cycle through the selector options on your HVAC control, feeling for cold air from each position. Positions that don't blow cold air may have a temperature sensor issue, while positions that don't blow any air might have a blend door actuator problem.
Question

How do I know if my AC fan is bad?

Ed Beery
Automotive Specialist
Ed Beery is an Automotive Specialist and the Owner of InTechgrity Automotive Excellence based in Denver, Colorado. With more than eight years of experience, he specializes in providing maintenance and repair services for both individuals and companies. Ed and the InTechgrity Automotive Excellence Team are approved by the American Automobile Association (AAA) for repairs and are Automotive Service Excellence (ASE) certified.

Ed Beery

Automotive Specialist

Expert Answer

Your AC fan will either have inconsistent air movement or no air movement at all.

Warnings

Do not add refrigerant unless you are sure that low refrigerant is the cause of the problem because overfilling the system with too much refrigerant can cause severe damage.

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It is always best to consult a professional to do repairs to your vehicle.

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Wear safety glasses and work in an open environment, where fumes won’t overwhelm you. Never touch your eyes or mouth after handling Freon or other chemicals. Wear long sleeves and gloves whenever possible.^{[16]
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