Car Overheat When AC Is On? Here Is Why!

Last Updated on July 20, 2025 by Azim Uddin

A vehicle overheating when the AC is turned on isn’t a normal thing. It is a sign there is something wrong with the car’s cooling system that needs to be fixed as soon as possible.

But first, why does the car overheat when the AC is on? Some of the common causes include the following:

• Faulty coolant temperature sensor
• Radiator fan issues
• A failing AC compressor overload
• A failing water pump
• Partially clogged condenser or radiator fins.

You now have a starting point. Thoroughly examine each potential cause in turn, and that should resolve the overheating issue. Read on to know what to do.

Why Does The Car Overheat When AC Is On?

No car owner wants to imagine replacing the engine because of overheating. So, taking preventive measures when signs of overheating start to show is vital. So, understanding why the car overheats when the AC is on qualifies as a priority before you hit the road.

But where do you start? Here are the few issues that are causing the problem for you and how you can fix them:

1. Faulty Coolant Temperature Sensor

A faulty coolant temperature sensor sends erroneous readings to the ECM. That interferes with the ECM ability to manage the cooling system properly. It gets worse when you turn on the AC. This puts an additional load on the engine from the compressor.

That increases heat generation in the engine that isn’t getting dissipated right, thus causing overheating. The most common telltale signs of a failing coolant temperature sensor are:

• Overheating engine
• Unstable temperature readings
• Check the engine light on.

Solution:

• Use an OBD2 scanner to inspect for consistent live temperature data from the CTS.
• If you’re getting unstable readings or none at all, replace the coolant temperature sensor.

2. Radiator Fan Issues

The radiator fan boosts the cooling system by facilitating efficient heat transfer from the coolant to the air when running. When you turn on the AC, the compressor places more rotational load on the engine, increasing heat generation. This puts greater thermal demand on the cooling mechanism.

If the radiator fan doesn’t kick in or turns on at the wrong time, this will lead to overheating. Some of the common issues that can prevent the radiator fan from turning on when needed include the following:

• Dead fan motor.
• A wiring radiator fan problem.
• A blown radiator fan fuse.
• Dead radiator fan relay.
• Failing radiator fan switch.
• Faulty fan control module.

Solution:

• With the engine running, turn on the AC to confirm if the radiator fan is running or not. If it is, move to the next possible overheating cause.
• If it is not running, you need to troubleshoot each of the possible causes mentioned above.
• Start by checking the fuse and the relay. Continue to inspect the wiring before testing the motor. If all these are working right, test out the radiator fan switch.
• Any part that you find faulty, whether the motor, fuse, relay, switch, or wiring, replace it.

3. Failing AC Compressor Overload

A compressor, the backbone of the car’s air conditioning system, can sometimes cause overheating. How? When the compressor starts to fail, it puts excessive rotational load on the engine shaft. This increased engine load strains the engine and causes it to generate more heat.

The extra load on the engine and increased heat generation form the basis for the engine to overheat when the AC compressor is engaged. If your car’s AC compressor is dying, you may experience the following symptoms:

• Ticking, chattering, rattling, or rumbling noise.
• High-pitched squeal or screech sounds.
• There is a grinding noise when you switch on the AC.
• Little to no cold air after turning on the AC.

Solution:

• Does your car show the signs mentioned above? If it is, take it to an auto shop near you for inspection.
• If the compressor is found to be dying, have it replaced before you get back on the road.

4. Failing Water Pump

A faulty water pump will lead to insufficient coolant circulation. As coolant absorbs heat from the engine, the pump pushes it to the radiator for heat dissipation and then back into the engine in a continuous cycle.

The extra demand for heat dissipation brought forth when you switch on the AC can’t be handled by the failing pump. Slowly, the heat builds up, causing overheating. It could be that it is worn out or weak.

The car overheating when you turn on the AC could be an early sign it is failing. Other symptoms include the following:

• High-pitched, harmonic whining noise.
• Coolant leaks around the pump.
• Coolant leaks into the oil

Solution:

• There is no repair for a failing water pump. You have to replace it with a new pump.

5. Partially Clogged Condenser Or Radiator Fins

If the condenser or radiator fins work by increasing the surface area for heat dissipation. However, they have one issue – they tend to get clogged by debris and other build-up.

When this happens partially, enough cold air does not get through, affecting the effectiveness of heat dissipation. If your condenser is installed in front of the radiator, it doubles the clogging problem.

The partial clogging might allow enough cold air to the radiator lines. However, when the AC comes on, the airflow might not be enough to dissipate the additional heat generated.

Solution:

• Brush the fins using a brush with moderately stiff bristle fins.
• You can also use compressed air to blow out the buildup gunk.
• If the build-up is stuck on the fins, use a radiator cleaning solution. It will help break down the gunk for easy cleaning.

FAQs:

Check the following frequently asked queries and their answers if you have any questions remaining about car overheats when the AC is on.

Q: What happens to the engine when the car overheats?

The engine may suffer from warped or cracked cylinder heads, damaged pistons, etc. These can lead to costly repairs or even engine replacement.

Q: What should I do if my car overheats when I turn on the AC in traffic?

If your car overheats while in traffic, find a spot and pull over. Shutting off the car allows the engine to cool for at least 15 minutes, and keep an eye on the temperature gauge. While waiting, don’t turn on the AC until you fix the problem.

Q: How much does it cost to replace an AC compressor?

The cost to replace an AC compressor can vary depending on the make and model of the car, as well as the location and availability of parts. On average, the cost can range from $500 to $1,000.

Conclusion

If your car is overheating when turning on the AC, it signals an issue with the cooling system that needs addressing. The most common causes are problems with the coolant temperature sensor, radiator fan, AC compressor, water pump, or clogged radiator fins. Thoroughly inspecting each component can help identify the faulty part requiring replacement to resolve the overheating problem and prevent more serious engine damage down the road. Taking early preventive action is important for the owner’s peace of mind and wallet.