Car AC Stops Working On Highway [5 Easy Solutions]

Car AC stops working on highway or under acceleration but it cools great at low speed! If you encounter this strange problem, you are not alone in the world. 

Like you, there are other car owners who also face the same issue. Do you want to figure out the possible reasons behind this weird problem? Then, continue to read till the end as we will disclose every culprit that causes your car AC to malfunction. 

car ac stops working on highway

Car AC Stops Working On Highway [5 Easy Solutions]

The car’s AC stops working on the highway due to several reasons. For example, the faulty thermal expansion valve is the main suspect if the car’s AC stops working at high speed. 

Therefore, a flaky clutch drops in refrigerant level, a vacuum leak, and loose electrical connections are other culprits that prevent your car’s AC system from functioning. Below, we will break down each of them.

Note: You can also read why is my car AC Compressor not turning off problem.

1. Faulty Thermal Expansion Valve

A defective thermal expansion valve is the first culprit to blame if your car’s AC system stops working at highway speed. This expansion valve controls the flow of refrigerant through the evaporator. If it’s at fault, it will freeze up AC systems.

checking thermal expansion valve

For example, if the thermal expansion valve gets defective, it will remain almost in the closed position and freezes up the orifice. Consequently, the TXV (thermal expansion valve) fails to control the right amount of refrigerant flow through the evaporator. 

And guess what, your car’s AC system won’t work and cool down the cabin if there is a lack of sufficient refrigerant.  

How To Fix:

Before replacing the thermal expansion valve, you should perform a pressure test and a performance test for the AC system. If your AC system passes this test, we bet the TXV is at fault.

It’s mounted to the evaporator and you can replace it separately. The following tutorial will help you in this case. However, handling this task with a professional will be the best bait. 

2. A Flaky Clutch

A cracking clutch is another suspect that prevents your car’s AC from working at highway speed. There should be a proper clutch gap to let the AC system work. Wear & tear clutch surfaces are also responsible for this issue. 

 A Flaky Clutch

How To Fix:

Start with checking the clutch gap and adjust it if needed by removing the shims. The gap should be 0.35. However, we recommend you check the specification sheet for your car’s make and model. 

On the other hand, if the clutch surfaces get cracked, it will be best to replace the clutch assembly. Also, don’t forget to check the low-pressure switch and the pressure of the AC system. 

3. Refrigerant Is Leaking

Refrigerant is a coolant that absorbs the hot air and cools down your car cabin. If the level of refrigerant drops, the AC system won’t cool down the air. Generally, the refrigerant level drops if there is a hole. 

checking refrigerant leak

If the shop doesn’t fix the leak, the refrigerant level will come down. After driving the car for a few hours, the AC system will stop working due to the leak of the refrigerant. 

How To Fix:

We recommend you run a leak test first to figure out the source of refrigerant leaking. In this case, you can use a sniffer tool or a UV tracer dye. This tutorial will help you use these two tools to find car AC leaks: 

Once you find the source of the leak, it’s time to fix the leak. Then, recharge the system by filling it with fresh refrigerant. Make sure you add the refrigerant to the AC system according to the manual. 

4. A Vacuum Leak

If the HVAC system of your car is controlled via vacuum, a vacuum leak can also be the culprit. A vacuum leak in the AC system will deplete the vacuum reservoir’s capacity.

check vacuum leak

Consequently, the HVAC system may manifest itself by the air doors moving to the default defrost setting under acceleration. 

How To Fix:

You must fix the vacuum leaks to overcome this problem. We highly recommend you watch the following tutorial which breaks down everything on finding and fixing vacuum leaks: 

Don’t hesitate to hire a professional to do this task for you if you are not mechanically inclined. 

5. Loose Electrical Connection

Loose electrical connections at the fan motor or within the blower motor resistor.

loose electrical connections at the fan motor

If the wiring to the fan motor comes loose, the fan will stop blowing air and your Ac won’t work. It will be more complicated if the fan starts blowing warm air. 

How To Fix:

You can find all the electrical connections behind the glove box and the front passenger’s toes. If you are not electrically inclined, you can hire a professional mechanic to do everything for you. 


Why does my ac only work on the highway?

Your car’s AC will only work on the highway or while driving if the AC condenser fan or its wiring gets damaged. Turns out, that a blocked AC condenser, a faulty air compressor, and a low coolant level are other culprits behind this AC problem. 

What does it mean when your AC stops working in your car?

The AC stops working in your car means there is a problem with the AC compressor. Therefore, leaking refrigerant, clogged air filter, dirty condenser, and filthy evaporator is also responsible for a malfunctioning AC system in your car. 

What to do if your car AC stops working?

If your car AC stops working, we recommend you to do a pressure test first. It will help you find the culprits that prevent your car AC from functioning. If you are not mechanically inclined, call a professional and let him deal with this problem. Never try to fix the refrigerant leaks by yourself. 


We disclosed all the possible reasons that cause your car’s AC to stop working on the highway or at full speed. Now, what you need to do is- just follow the troubleshooting steps and fix those problems. If you are not mechanically inclined, we recommend you call a professional for help. 

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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