Why Does My Car Overheat When the AC Is On?

If your AC is cooling you down but heating up your engine, here are some possible causes as to why your car overheats when the AC is on.
Written by Natalie Todoroff
Reviewed by Melanie Reiff
One of the most common reasons why your engine overheats when your AC is on is a faulty engine cooling fan, fan switch, or fan motor. A bad engine coolant sensor, an overloaded AC compressor, a broken water pump, and blocked AC condenser fins can also create overheating issues.
On a hot day, your car’s AC system could be doing a fine job keeping you cool and comfortable in the front seat—but there could be a different story going on inside your engine. If you’ve got the AC blasting and notice that your engine’s temperature gauge is creeping up, you’re probably scratching your head about what could be causing the issue. 
Your engine is complex, so if you’re feeling overwhelmed trying to pinpoint the root of the problem, let
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Reasons your car overheats when the AC is on 

If you’re completely stumped as to why your car overheats when the AC is on, below is a list of some of the most common causes of this particular kind of hot mess. 

Bad engine coolant sensor

The engine coolant sensor is a pretty self-explanatory part of your vehicle: its job is to monitor your engine’s internal operating temperature. It reports the car’s temperature to the Electronic Control Module (ECM), which (among other things) determines when to turn on your engine’s cooling fans. 
If you have a defective engine coolant sensor, it will report an inaccurate internal engine temperature to the ECM. This can cause the fans to not turn on when they should, which can lead to your car overheating. 

Overloaded AC compressor 

AC compressor
is an absolutely vital part of your car’s broader AC system. Its job is to pressurize your engine’s liquid refrigerant and draw power from your engine’s output shaft in order to do so. 
If your AC compressor is malfunctioning, it puts what’s known as a rotational load on the engine shaft, which in turn leads to your engine overheating. 

Defective water pump

faulty water pump
could also be to blame. It circulates a mixture of coolant and water throughout your engine in order to keep it at a healthy internal operating temperature. 
Without a working water pump, none of your car’s cooling systems will work—which leaves both you and your engine uncomfortably hot. 

Bad fan, fan switch, or fan motor

Bad engine fans are one of the most common causes of why your car overheats when your car’s air conditioning system is on. 
These fans aren’t the ones that blow the cool air onto your face when you flip on your car’s air conditioning system. Rather, they pay attention to your car’s airflow in order to adjust their fan speed to cool the engine as needed. 
If the engine cooling fan is defective—or if the fan blades are bent out of shape, the fan switch that triggers the fans to run, or the fan motor that powers them is not working—then you can experience an overheating problem. 

Blocked water condenser fins 

While AC condenser overload can cause your car to overheat, it’s more likely there's an issue with the condenser fins. If you’re not familiar with the AC condenser, it’s where hot refrigerant cools off to be distributed throughout your engine. 
A lot of dirt and debris can wind up stuck between the condenser’s fins. When that happens, the AC condenser will not work as efficiently, and the coolant mixture will not be able to cool down as much. The result? Your car’s engine will overheat when you turn on the AC system. 
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What are the parts of air conditioning in cars?

Your car’s HVAC system is notoriously complex. Reading about some of the parts above may have left you feeling a little mind-boggled, but don’t sweat it. 
In general, your AC system works as a sealed, closed-loop system in which refrigerant is used to lower the temperature of the circulating air. So, if you’ve got an overheated car, something in the coolant system has failed. 
Here’s a brief rundown of some pivotal parts of your car’s air conditioning system. 

Cooling fan 

Powered by a fan motor, the cooling fans adjust their fan speed in tune with the internal temperature of the engine. 
When the internal temperature of your engine leans warmer and there’s too much hot air, the cooling fans speed up. And, on the flipside, they slow down when the engine’s temperature is closer to normal. 

Water pump

The water pump gets its power from your car’s crankshaft. It’s easy to get this part confused with an
oil pump
, but they are completely separate parts. An oil pump circulates engine oil to lubricate your engine, while the water pump pumps a coolant mixture into the radiator. 


The radiator is a liquid-to-air heat exchanger that helps cool your engine. The radiator is made of a system of tubes (among other parts), that receives liquid coolant from the water pump. As the coolant mixture flows through the radiator, the radiator blows air across it to cool it down before it’s released back into the engine. 


No, this one isn’t the one that your dad wouldn’t let you touch in your home. Your car’s thermostat controls how often the coolant can “cycle” off and on in response to changes to your engine’s temperature. 

How to fix an overheating engine when the AC is on

Check cooling system

And for this, we mean you should
open the hood
and take a look at your radiator—a clogged radiator could be the cause of your overheating troubles. Take a look at the radiator hose and make sure it’s free of any kinks or coolant leaks. 
But if those parts look fine, you should grab your mechanic’s creeper, get under your vehicle, and check on the radiator cap. If you’ve noticed that your radiator is steaming and fogging up your front windshield when you drive with your heater at full power, it’s likely that your radiator cap is jammed shut. 

Refill coolant 

Thankfully, this is an easy DIY fix. Low coolant levels are also one of the most common causes of why your engine is overheating. Locate your coolant reservoir and make sure that’s filled up to the fill line. In many instances, topping off your coolant levels can fix the issue! 
Now would also be a good time to take a closer look at your coolant reservoir for any coolant leaks. If you spot any, you may have to get a
coolant reservoir replacement
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Check refrigerant 

We touched on your car’s AC compressor system earlier, but it’s worth revisiting here. When refrigerant inside of the AC system is not properly pressurized, your AC system has to work extra hard to lower the temperature—which results in (you guessed it) your engine overheating. 

Have your air conditioner serviced 

There’s really only so much DIY diagnosing that you can do. 
If you’ve checked out all of the parts listed above and still can’t figure out what’s going on with your AC system, it’s time to
take your car in to a professional
For this, we’d recommend finding a mechanic that specializes or is certified to repair car HVAC systems. As we mentioned earlier, they’re tricky beasts, and you’ll need someone with the expertise and credentials to make sure you’re fixing the problem. 

Get parts replaced 

And, if all else truly fails, you may have to swap out a couple of parts if they’ve become faulty. Exactly how much you’ll have to shell out depends on the part you need to replace: 

How to find affordable car insurance 

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A faulty AC compressor in and itself cannot cause the engine to overheat. If the compressor clutch is engaged and the AC compressor is on the fritz, it will cause excess strain on your engine. It’s this excess strain and effort that causes the engine to overheat, not necessarily the compressor itself.
If you’re running your AC while idling and your engine keeps overheating, there’s most likely a problem with the AC compressor clutch, compressor pulley, or AC compressor.
A red light on your AC compressor indicates that the compressor part is no longer working or is near the end of its lifespan. Either way, you should have a mechanic take a better look at it—it’s not a good omen!
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