Why Does My Engine Stall When the AC Is On

If your car keeps stalling after you turn on the A/C, there could be a few potential causes for this. It could be a problem with your idle air control, a power issue with your alternator or battery, or even a problem with your A/C itself.

Faulty IAC

The IAC (idle air control) is a valve that is connected to the throttle body and is pretty significant when it comes to helping your engine run properly. As you may know, your engine needs a steady supply of air to run. Usually, this air supply is regulated by the throttle, which is controlled by operating the gas pedal.

However, the throttle is closed when the engine is idling, so there needs to be a way for the engine to receive air aside from opening the throttle, and this is where the IAC comes in. The IAC bypasses the throttle and sends a small amount of air directly into the engine, which is what allows it to run at idle speeds when the gas pedal isn’t being pressed.

If the IAC fails, this can significantly affect the engine’s ability to idle correctly. At best, it may idle a lot rougher than normal, or the idle speed may be noticeably irregular; at worst, a faulty IAC can cause your engine to stall unexpectedly.

In particular, if you have a faulty IAC and you put your engine under an increased load (for example, by running your A/C), then there’s a good chance that your engine will stall immediately. This is because your engine isn’t receiving enough air to compensate for the increased power drain.

Carbon deposits can build up in the IAC, and these deposits are often the cause of a bad IAC valve. Luckily, this is a pretty easy fix, and you can remove these carbon buildups by using a fuel system cleaner. 

Faulty Battery/Alternator

A faulty battery or alternator can easily cause your engine to stall when coupled with the extra load from a running A/C system. 

If your battery is running low on power, your alternator will be forced to work harder to recharge it, and this, of course, puts a greater load on the engine. If you run your A/C at the same time that your alternator is trying to recharge your battery, this can potentially overload your engine and cause it to stall.

Similarly, if your alternator itself is starting to fail, this can cause your engine to stall if you try to run your A/C thanks again to the increased load on the engine. 

If you’re unsure of whether your engine stalling is being caused by an electrical issue or not, one thing you can do is use a multimeter to test your battery’s power output. If the multimeter shows you a result that is lower than what your battery should normally be putting out, this could very well be the cause of your problem.

If your battery is holding its charge but your car is still experiencing electrical issues, this could be a sign that the problem lies with your alternator. 

Bad A/C Compressor

If your car is running fine in general but keeps stalling only when you turn on your A/C, this could be a sign that the problem lies with the A/C system itself.

If you don’t know how your car’s A/C system works, the basic idea is this: the A/C system contains a refrigerant, which is normally in a gaseous state. The refrigerant is compressed into a liquid by a compressor, which cools off the refrigerant.

The cooled refrigerant is then returned to its gaseous form and is sent to a receiver/drier, which removes moisture and other impurities from the refrigerant. Finally, the refrigerant goes to the evaporator, where it pulls heat from the surrounding air. The resulting cool air is then blown into the cabin while the reheated refrigerant cycles back to the compressor to be cooled again.

If your A/C compressor isn’t working properly, however, this can cause it to overload the engine. There are a few reasons why your compressor might stop working; the internals might be worn down, it might not be properly lubricated, or there could be too much refrigerant in the system which the compressor simply can’t handle.


Cars can be pretty complicated, and if you’re not someone who is not all that mechanically knowledgeable, it might not make any sense to you why running your A/C would make your engine stall. 

If you’ve spent a lot of time working on cars before and you’re confident of fixing problems like these on your own, it can be pretty simple to deal with such issues. Otherwise, we’d suggest taking your car into a repair shop if the engine keeps stalling unexpectedly, as there are a lot of reasons why this could occur.