Advance Answers: Questions About Vehicle A/C Systems

car air conditioner pictureWe explore a couple questions that have come our way regarding your car’s thermostat and how vehicle air conditioning systems work.

Updated: May 2016

In general, using logic and common sense when diagnosing problems with your vehicle — and when making repairs and addressing issues — is a good strategy. You eliminate what doesn’t make sense as you narrow down your diagnoses. Every once in awhile, though, what you should do is somewhat counterintuitive. Especially when it comes to vehicle A/C systems (which are part of a vehicle’s HVAC system, or Heating, Ventilation, and Air Conditioning).

What should I do when my car overheats?

Sometimes cars overheat (fortunately, not as often as they used to). This happens most often in hot summer weather — and if you’re in the car, you’re probably feeling pretty toasty yourself.

The first thing to do is to shut off your car’s air conditioning and open the windows to decrease the burden on your engine.

So far, that makes sense — but, the counterintuitive part is this: if your car continues to overheat, then turnon both your heater and its blower. It may be uncomfortable, but this transfers heat away from the car’s engine into the inside of the vehicle itself.

Why should I use my A/C system in the winter?

The best thing to do during winter is run your A/C. It sounds strange, but running your vehicle’s air conditioning system during the winter can make sense (just remember your gloves!). If you run your A/C throughout the year, the system stays more lubricated, helping to prevent leaks. That’s because the refrigerant contains oil that lubricates the system, including the compressor. It also keeps seals and hoses moist, which helps to prevent the dryness that leads to cracks and leaks in the system.

Your car’s A/C system is also much more efficient at window defogging than the heating system. So, turn it on to clear up the fog — and, if your A/C doesn’t do the job, check the compressor because this might indicate a problem.

How important is it to fix the thermostat?

The thermostat is a relatively small and inexpensive part with an important job: it senses and “reports on” the varied temperatures throughout the vehicle’s engine. The engine needs to run hot to burn fuel, but if it gets too hot, the thermostat signals the release of coolant to reduce the temperature.

If the thermostat isn’t working properly, the coolant can keep flowing until it’s all burned off, which can lead to overheating or even more severe problems, like a blown cylinder head gasket.

In cold temperatures, the thermostat prevents water from going to the radiator. This helps the engine warm up enough, even on bitter winter mornings.

If you think your thermostat might be operating at less than optimal efficiency, it’s often easier to replace it to see if that solves your problem, versus going through more complicated diagnostics. Check the original thermostat in your vehicle and buy a comparable thermostat replacement. You can also check your owner’s manual to see which type of thermostat the OEM recommends.


  1. The car’s AC unit is a vital part of the vehicle. I had never thought about the benefits of running the AC during the Winter, but it does make sense. It seems that most problems happen in the Spring, after months of not being used, so thanks for the tips!

  2. Brenda Gideons says:

    I have been in the parts business since 1972 and I still learn something I didn’t know just about every week. I can do simple repairs, like changing light bulbs, wiper blades, batteries, change fuses, just easy stuff. I’m a 62 year old female. I’ve learned a lot listening to mechanics tell people what may be their car’s issue. The customer returns stating that was the problem. Finding information here is also helpful in relaying it to my customers . Thanks

  3. I recently had the vacuum pump replaced on my 2000 S-10 blazer 4×4 and my heater doesn’t blow through the vents, but the A/C works perfectly. What about the heat?

    • Sorry to hear that Alvin. I will need to know a little more information. Does the A/C work perfectly though the vents and then when you change the temperature control to heat instead of cool it just stays on cold air? If that is the case, it sounds like the heater valve or blend door is malfunctioning.

  4. Jim Ponder says:

    I can speak with 25 years of experience. Change the thermostat every few years. Just changed mine to extend the life of the engine. Helped with fuel mileage as well. I would run 190 degrees thermostats all the time even if the original calls for something higher.
    A/C running monthly should be enough.

  5. Can the A/C, while on, cause the vehicle to idle roughly while in gear? Didn’t start until the last year and came on gradually. I can shift into Park or Neutral and it runs smoothly. Turning off the A/C also eliminates the rough idling. With A/C off, the car just purrs. Could this be a sign of low refrigerant even though the air is still cold? 04 Toyota Echo 60K. Advice or ideas appreciated!

    • Yes, this certainly can happen. What is causing this is that the A/C compressor is putting a load on the engine and therefore causing more vibrations through the car. If you are driving and stop on an incline, you will probably notice it even more. But the fact that it just started happening this year is not normal. I would check the belt tension on the A/C serpentine belt. Many times belts stretch over time (they are rubber after all) and will cause a looseness when a load is placed on them. By replacing the belt and making sure there is proper tension on the system, it will eliminate any vibrations from stretching.

  6. Why is the article titled about A/C then it doesn’t talk about A/C but about engine cooling. I want to cool myself in the car thus A/C.

  7. Calvin Moore says:

    My A/C dose not work in the drivers area but works in the rear.I have a chevy travsiers 2010

    • Hey Calvin, this is actually a pretty common issue. The problem is there are sequences of Blend Doors that actually control the routing of airflow in your vehicle. Usually this is front to rear, left to right. You will have to have a better diagnosis from a mechanic as to which one is faulty, but the blend door behind/under the dash is the issue.

  8. Deardre Morgan says:

    This was vital and important information , towards savinhume hundreds of dollars instead of purchasing a new A/C system. I wish this information was notice two weeks ago. Thank you for the information because this will help me with our new car.

  9. I’ve got a 2002 GMC Sierra that just won’t blow threw the dashboard for nothing. It’s blowing ice cold on the floor and defogger. Any opinions for me.

    • Hey Anthony, this is actually a pretty common issue. The problem is there are Blend Doors that actually control the routing of airflow in your vehicle. Generally you will need to know if you have manual or automatic controls to determine which blend door you need. You should have a diagnosis from a mechanic as to which one is faulty, but the blend door behind/under the dash is the issue.


    With newer cars your a/c automatically comes on with the defrost

  11. What would make my 2008 Chevy cavalier, have a fan running after car is shut off? Also there’s a clicking when I turn the A/c on or defrost, or air coming in from outside! Sounds like it’s in the dash, when have it on face with a/c off, just fan blowing, no clicking!

    • Hey Candy! Since they quit making the Cavalier in 2005, did you mean the Cobalt? Typically the reason why the fan is running even after the car is shut off is because the blower motor relay is stuck closed and the relay needs to be replaced. For your second issue, sometimes debris can get behind the dash into the vent system. This could be leaves, pine needles or even small twigs. I would check the cabin air filter and make sure that is clean as well.

  12. Lewis Campbell says:

    Why would in a car with duel hvac controls ,
    Like my wife’s 2004 Honda Accord will one side cool real nice while the other side blow warm and sometimes hot air ?

    • Hey Lewis, this is actually a pretty common issue. The problem is there are blend doors that actually control the routing of airflow in your vehicle. Generally you will need to know if you have manual or automatic controls to determine which blend door you need. You should have a diagnosis from a mechanic as to which one is faulty, but the blend door behind/under the dash is the issue.

  13. On chevrolet tahoe 97. Ac is working I installed new blower motor and blower motor resistor. I check the ac if need some. But shows like is filled. Ok. It takes like 10 or 15 minutes to get cold inside. Is the weather or I need to check something else. Do they have ac filter inside or not

    • Hey Jesus. Could you check your refrigerant levels again with the setting on “Max A/C” and the blower motor on high? Make sure the compressor is engaged and working. Then put the refrigerant level gauge on the low pressure port of the system and check the level. If it’s low, add more refrigerant into the system. If it’s full or high, you will need a mechanic to give you a better diagnosis.

    • If the ac is blowing warm or barely cool air and the pressure gauge shows it’s full, the problem could be there is air in the lines that need to be bled out. It’s easy to do. simply open the low port where you fill the refrigerant and press the valve down. Air will come out. When the coolant begins to surface, stop. Go and start your car with the a/c running. Let it run for approx. one minute. Shut the engine off. Wait a minute for the air to resurface then release more air out of the port. Repeat several times. If the air gets colder you are on to something. Check the air gauge, if desired and add more refrigerant as needed.

  14. JDizzle says:

    I have a 95 GMC Sierra that I put new AC compressor & hoses on last summer. My clutch on the compressor won’t kick in, I try to give it freeone & the gauge goes in the red. What should I do?

    • Hey JDizzle. I know this is not the greatest news, but we’d recommend bringing it into a professional. There could be a number of issues like the compressor malfunctioning, the filter dryer needing to be replaced, or even the expansion valve. Also, ’95 is a year that could have had R12 or R134a refrigerant. So unless you did the conversion properly, that could be another issue with your A/C. (Nice name, by the way)

  15. How does a DIY type check out am A/C compressor ????? Other than seeing if the system works when it has refrigerant, what can you do ?????? OK: see if the clutch is activating……… But without any pressure readings, how can you tell if it is working ?????

    Also, your explanation re thermostat malfunctioning seems off base…………………e.g : ” the coolant can keep flowing until its all burned off……………” implies boiling off the coolant, but your context is along the lines of the thermostat opening/staying open too much. While coolant can indeed be boiled off, this is not due to a wide open thermostat per se.

  16. Bret Paolino says:

    I am an ASE certified auto repair/parts technician with 30 years experience.
    Most vehicles produced with A/C in the past 20 years have the compressor cycle while the defroster is on so the Freon and oil will circulate through the system.

  17. My car aircon blows cold for half hr them blows normal temp. it will not return to cold air until you park and everything cools down. Whats the problem?

  18. Thank you for these great tips. I must say that I am quite surprised by the second point. I never knew that switching on the A/C during winter was necessary I will surely implement these tips.

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