6 Reasons Why Your Car Overheats When AC is On + Solution

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car overheats when ac is on

Has it ever happened to you that your car was driving just fine, but as soon as you turned on the air conditioner, the temperature gauge crossed the allowable limit? Indicating that your car overheats when ac is on.

It’s normal for some cars to reach an elevated temperature for a short period of time while the engine is running and the air conditioner compressor is on (it takes more power from the engine to compress the refrigerant). But when the compressor is off, it shouldn’t get hotter than usual.

Why does it happen? Before you can understand this, prior knowledge about two things is required. One is the air conditioning system of your car, and the other one is the coolant system.

Air Conditioning System in Modern Cars

air pressure gauge

Automobiles often have their own interpretation of an air conditioner which works about the same as far as cooling goes but may not have all the additional components that a full-size AC system would have.

The main components are the compressor, condenser coils, expansion valve, evaporator coils, and receiver/drier. All of these components work on the same basic principle of the refrigeration cycle.

The air conditioning system is a closed-loop, sealed system that uses a refrigerant to “condition” the air inside of the vehicle.

The compressor changes the low-pressure gas from the evaporator into a higher-pressure gas that can be moved throughout the system under pressure. It then goes to the condenser coils and gives up its heat to the condenser coils outside of the heated space.

The refrigerant then goes on to the expansion valve, where it undergoes a pressure drop and becomes a low-pressure liquid that is sent back through the evaporator again.

These common components work together to cool you off by removing humidity from the air inside of the vehicle. In the summer, humidity is a hot commodity that can make your car feel much hotter than it really is when it comes to how “comfortable” you are in the car.

Cooling System of a Car

car air conditioning

Before we dwell on why a car overheats when ac is on, you must know how a car’s cooling system works. Car overheating is a direct result of the coolant system failure.

In a car, various components work together in order to regulate the temperature within the cabin. The cooling system works alongside these components to maintain a constant temperature while driving in summers and avoid the extreme heat that can potentially damage your car.

Cooling Fan

The cooling fan aligns itself with the car’s airflow so that it can speed up or slow down according to the temperature of the engine. This ensures that the engine is cooled at all times. The cooling fan speeds up when the interior of your car becomes hotter than normal and vice versa. It ensures that the engine is cooled at all times so that it does not overheat.

Water Pump

The water pump in cars is the engine’s drive unit, driven by a belt from the crankshaft. This pump supplies water to the cooling system and does not have any effect on engine oil lubrication, which takes place in a separate oil sump.

The pump has a filter through which the coolant passes before going into the radiator.

The fluid inside your car’s engine is maintained by a pump that gets its energy to work from the pressure created by the movement of rotations caused by the engine’s rotation.


A liquid-to-air heat exchanger often called a radiator, is one of the most basic ways to remove waste heat from an engine.

A liquid coolant passes through tubes in the radiator, and the coolant exhausts its heat to the inflowing air as it passes over fins that extend into the airstream. The air is then expelled out the back of the car, and the coolant is returned to the engine.

The engine coolant is a mixture of water and ethylene glycol or water and propylene glycol. So, the fluid going back into the engine is a mixture of these two chemicals in a certain proportion.


The thermostat is one component of your car’s cooling system that works to maintain the engine temperature. The thermostat responds to changes in coolant temperature by regulating how often the coolant system is allowed to “cycle” on and off.

If the thermostat fails, it may either open too quickly or stay open too long, resulting in overheating the engine.

Why My Car Overheats When AC is On?

If your car overheats when ac is on, it might be due to the following reasons:

  1. Defective Engine Coolant Sensor
  2. AC Compressor Overload
  3. Faulty Water Pump
  4. Faulty Fan, Fan Switch, & Fan Motor
  5. Clogged Condenser Fins

Let us explain these reasons a bit:

1. Defective Engine Coolant Sensor

The coolant temperature sensor’s job is to keep track of the engine’s operating temperature. The ECM (Electronic Control Module) uses this information along with outside air and intake temperatures to determine critical engine parameters. The ECM cannot properly function without the coolant temperature sensor.

If the engine coolant temperature sensor goes bad, it will report the wrong coolant temperature to the ECU. As a result, the coolant system’s working is compromised, and you might experience that engine overheats when ac is on.

2. AC Compressor Overload

A compressor is the backbone of the car’s air conditioning. The compressor’s main job is to increase the pressure of the refrigerant. A failing ac compressor would not be able to do the job correctly.

The ac compressor draws power from the output shaft of the engine. When the compressor goes bad, it puts a rotational load on the engine shaft. An increased engine load is the basis for engine overheating.

Hence, you must lookout for a mechanic in this scenario and get your ac compressor repaired.

3. Faulty Water Pump

If your car regularly overheats when ac is on, then it might be due to a weak or failing water pump. The job of the coolant pump is to maintain the engine temperature. It does so by pumping the coolant mixture in water jackets around the engine compartment.

A bad coolant pump results in an incompetent cooling system. If the engine cooling system fails, you will get deprived of cool air in the car’s cabin.

Thus, if your car overheats when ac is on, you must check the pump’s health.

4. Faulty Fan, Fan Switch, & Fan Motor

Cooling fans ensure the smooth heat transfer between the engine coolant and the outside air. When the cooling fans of your car are not working correctly, you will experience overheating problems.

A faulty engine fan is also the leading cause behind engine overheating when ac is on. Car ac system puts extra strain on the engine. As a result, the heat is being generated at higher rates.

The coolant system has to work even harder to remove the excessive heat. When the aiding component, like the cooling fan, goes out, the chances of car overheating are multiplied.

Different problems with cooling fans can cause car overheating. Like the blades of the fan may have become distorted. Or the fan switch that is responsible for turning on the fan might have gone bad. Similarly, the failure of the cooling motor can also be the culprit.

5. Clogged Condenser Fins

AC condenser is the component where hot refrigerant loses its heat. As a result of this, cooling occurs. If your car overheats when ac is on, it might be a direct result of the condenser failure.

But a more common reason is dirt and debris trapped in the condenser’s fins. If the fins on the condenser are clogged, then the heat transfer process would not be as efficient.

This puts extra pressure on the air conditioner, resulting in an overheated engine.

How To Fix Engine Overheating When AC is on?

If your car overheats when ac is on, you can avoid it through various means. You have to follow the advice given below, and most probably, your issue will be resolved.

Check the Cooling System

If you think your car’s engine is overheating, the first thing you should do is look at your cooling system.

There are several parts to your cooling mechanism, but we’ll focus on the radiator and hoses for our purposes here. Your radiator needs to be in good condition and not clogged. The hoses should be supple and free of leaks etc.

If you’ve checked your cooling system and it’s still overheating, the next thing to do is get under the hood of your car and take a look at your radiator cap.

When you see steam coming up from the radiator, before it gets to the top of the engine compartment (i.e., in front of the windshield) and when you drive slowly for a while with your heater on full blast – it probably means that your radiator cap is stuck shut and not working properly.

Refill Coolant

Most of the time, the problems related to the cooling of the car are caused due to low coolant levels. All you have to do is check the coolant on time and refill it according to the need. Do this, and you can avoid car overheating.

If your coolant level is reduced more than usual, then you need to check for a coolant leak as well. If a leak is found, you must flush out the old coolant, fix the leakage, and pour in the new coolant.

Check Refrigerant

If your car overheats when ac is on, you must check the pressure of refrigerant in the ac conditioner of your car.

If enough pressure of the refrigerant is not maintained, the ac has to work extra hard to achieve the desired temperature. It draws more power from the engine shaft, due to which you may experience car overheating.

Service Air Conditioner

Get your air conditioner serviced if your car overheats when ac is on. A timely service rules out the possibility of clogged condenser fins, a clogged radiator, low refrigerant or coolant level, and many such issues.

Hence, if your car overheats when ac is on, you can be sure that it is not due to the above issues. You must start looking beyond this scope.

Parts Replacement

If no above solution is doing the job for you, then the only viable option you are left with is the replacement of potentially defective parts. You can try replacing the following parts in order to avoid car overheating:

  • AC Compressor Replacement – $1,200-$3,500
  • AC Condenser Fan Replacement – $200-$900
  • Water Pump Replacement – $200-$1,300
  • Coolant Temperature Switch (Sensor) Replacement – $100-$500

Any of these replacements might solve your issue of car overheating.


Any car overheats when its coolant system is not working efficiently. If your car overheats when ac is on, it might be due to several reasons. But it would come down to one and one thing only. Which is that turning on the AC had put extra strain on the engine output shaft.

The increased rotational load also caused an increase in the heat generation rates of the engine. The cooling mechanism was not able to remove this excessive amount of heat. And this resulted in an overheated car.

Apparently, car overheating might be caused due to a failed water pump, a bad coolant temperature sensor, clogged radiator, clogged condenser fins, a faulty cooling fan motor, or a faulty ac compressor clutch.

Most of these problems are solved during the cooling system inspection and servicing of the air conditioner. Keep your coolant and refrigerant levels up to normal and avoid cooling problems in your vehicle.

Frequently Asked Questions

Can your AC make your car overheat?

Yes, your car engine may overheat when the ac is on. It is mainly due to the fact that the ac compressor draws power from the engine shaft. This means added pressure on the engine.

If more power is generated in the engine, it means more heat will be wasted as well. Now the cooling components might not be able to remove this excessive heat, and the car overheats as a result.

Why does my temperature gauge go up when my AC is on?

As mentioned above, the car engine may also become overheated when AC comes on. However, you should not rely only on the temperature gauge going up. A temperature gauge might go up due to a malfunctioning coolant temperature sensor as well.

Why does my car overheat when idling with the AC on?

It is mostly due to a faulty cooling fan. As the cooling fan is responsible for enhancing the heat transfer between hot coolant and outside air. When it goes bad at idle, there is no air blowing past the radiator and condenser fins. As a result, the car overheats.

What are the symptoms of a bad AC compressor?

If the compressor of the air-con system of your car has gone bad, then you might experience the following symptoms:

  1. Car engine shut down
  2. Ticking sounds coming from the engine compartment
  3. Temperature gauge going up
  4. Strange odor from the cabin
  5. Sluggish acceleration

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