Why Does My Car Idle Rough With AC On? (7 Reasons Why)

It is normal to experience lower RPMs when you turn on your car’s AC unit, but the RPM tends to go back to normal afterward, but sometimes it does not, and your car idles rough.

If this happens to you, you may be wondering why your car idles rough when the AC is on. We did some research, and here is what we discovered!

Why Does My Car Idle Rough With AC On?

A damaged AC compressor is the main reason your car idles rough when you turn on the AC. Plus, a dirty throttle body can make your car vibrate when you switch on your AC unit. Faulty parts like spark plugs, fuel injectors, and fuel pumps can make your car idle rough when the AC is on.

Are you curious about learning more reasons why your car idles rough when the AC is turned on? We made a list of factors below, so read on!

1. Damaged AC Compressor

An AC compressor is the main part of your car’s AC system since the AC compressor circulates refrigerant throughout the AC system.

However, a damaged AC compressor adds more pressure to your car’s engine when you turn the AC on, making the car excessively vibrate.

Generally, AC units need a lot of energy to function properly, so lacking power will make the AC unit not work as expected.

Additionally, defective AC compressors can send debris into the AC unit’s system, causing more damage that can further make your car idle rough.

You can check if the AC compressor is the cause by turning on the AC unit while accelerating the car to see if there is a difference in how your car idles.

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2. Dirty Throttle Body

Generally, a throttle body ensures there is proper airflow throughout the engine of the car, which can quickly catch dirt since air passes through the throttle body regularly.

When the car is idle, the throttle plate should be closed while the car’s computer regulates airflow over the car’s throttle body.

However, the throttle body will not work properly and add more pressure to the engine when the throttle body is full of carbon and dirty, causing your car to idle rough.

Normally, you would know that the throttle body is the problem if your car vibrates whenever you turn on your AC and potentially stall the car when you attempt to jump-start it.

Typically, the most effective method to fix this problem is by replacing the old throttle body with a new throttle body.

3. Bad Cooling Fan Or Condenser

A condenser is in charge of releasing heat from the refrigerant from any air that passes through the evaporator then the airflow is made by the cooling fan.

Generally, the condenser and cooling fan are crucial components that you will find in every car AC unit.

However, if the cooling fan or condenser is defective, it will increase pressure on your car’s engine, which will lead to a rough idle.

If your cooling fan is the issue, make sure that the cooling fan runs properly if your cooling fan possesses a variable speed.

Also, if you think your condenser fan is the issue, you can do some basic troubleshooting methods or check the circuit breaks.

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4. Faulty Spark Plugs

Faulty Spark Plugs

Another common reason your car shakes whenever you turn on your AC is that your spark plug is faulty.

Spark plugs supply sparks that ignite a mixture of air and fuel to make an explosion that helps your engine create power.

However, if the spark plug is damaged, worn out, or old, it may not be able to give the engine the spark it requires.

If this is the case, there will not be enough power throughout your engine, so turning on your AC, which requires a lot of power, can make your car idle.

Generally, the only solution to fixing this problem is by completing replacing the defective spark plugs with new ones.

5. Faulty Fuel Injector

The fuel injector is a crucial component of your car’s engine since the fuel injector controls the flow of fuel in the car.

Since your fuel injector controls electronic valves, it opens and shuts several times within a single second.

Therefore, if your fuel injector becomes damaged, the fuel injector will strain your engine, creating rough idling whenever you switch on your AC unit.

6. Bad Fuel Pump

Another common reason your car shakes or stalls when you turn on the AC unit is that your fuel pump is defective.

Typically, your car would rough idle or suddenly stop once you turn on your AC when your fuel pump is worn out or damaged.

Usually, you will just need to replace the fuel pump in your car to allow fuel to properly flow throughout your engine.

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7. Low Engine Battery

As stated earlier, the AC unit can take a lot of electricity to run, which can cause various issues.

That said, turning on your AC unit is more likely to make your car idle rough when your engine battery is low.

While your car normally relies more on the alternator, your car will likely have to turn to your car battery when you turn on your AC, which adds more load to your car’s battery.

Also, when your AC uses your car’s battery with the extra load, your car’s battery will likely quickly drain.

Generally, your car will idle rough and frequently stall when your battery is low because there is not enough power to sustain the AC and the car.

With that, you can avoid this issue by making sure the battery has enough charge and that your alternator works properly.

To know more, you can also read our posts on why your car idles low, why your car idles high, and why your car goes into gear but not move.


Generally, cars idle rough whenever the AC is on because there is an issue in the AC compressor or throttle body.

Additionally, a car’s AC uses a lot of electricity, so the problem can be caused by a low engine battery.

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