AC Blows Warm Air In Car – 10 Causes and Fixes

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Is hot outside and your AC blows warm air in car? Then your summer might not as pleasant as you have imagined. Although you can easily fix it by refilling the refrigerant level, it might happen that the compressor is broken since you did not use it in a long time.

So why is blowing warm air inside the car? The most common reasons that AC blows warm air in car are leaks in the air cooling system, broken compressor, worn-out clutch or electrical wiring problems. Each of these problems has its own level of gravity thus different costs of repairing.

How AC system is working?

When you hit the AC on the button inside your car, the compressor engages. This will start the gas pumping through the pipes of the system.

The compressed air, it’s usually at a high temperature with high pressure (the pressure usually depends on the outside temperature).

Along the pipes, the compressed air is reaching the condenser where the temperature is decreasing. At the condenser’s exit the compressed air takes a liquid form and as a result the temperature is almost half of the initial value. In other words, that is the role of the condenser in the system.

The newly formed liquid is circulating through the dryer to eliminate any form of gas, because is not wanted in this part of the system. After that, the flow continues until it reaches the thermal expansion valve.

Here the pressure drops approximately 10 times lower. The high-temperature liquid can escape through a small hole, therefore transforming it into a gas. Here is where actually the temperature drops in the 0 degrees range.

Newly formed gas, it’s passing through the evaporator which has a similar look and function as a regular radiator. Heater fan blows the air from the outside over the evaporator which cools down the air that it’s entering the cabin.

That cold gas travels back to the compressor where the whole process starts again. That’s how you get chilly air inside your car!

Check also a nice video which is showing the process discussed above:

Why AC blows warm air in car?

There might be several reasons AC blows warm air inside the car, based on the above explanation. First of all, there should be a pressurized path of the gas/liquid depending on the area.

If the pressure drops along the way, the exchange from high pressure to low pressure, it’s not properly done.

Secondly, all the components should function accurately. The most important and also the most expensive it will be the compressor which can keep the flow going. For instance, a broken compressor it’s not able to keep a constant pressure in the system.

Thirdly, there are electrical wires and connectors that might cause intermittent issues. For instance, if the compressor is not engaging when you press the AC ON button and everything else, it’s excluded.

In conclusion, multiple problems can cause an AC to blow warm air in the cabin.

10 causes why AC blows warm air

1. Refrigerant level low

A refrigerant is a fluid that is transformed from liquid to gas and vice versa in a cooling system.

Like we talked about, to have a good cooling inside the car and not the undesirable hot air, we need a constant pressure. 

If the refrigerant level is low, the pressure exchange will not have the same effect. The result will be an air inside the cabin beyond expectations. Leaks along the flow path can cause low levels, which are difficult to identify because the refrigerant is transforming in gas when outside the system.

2. Defective compressor

We can call the compressor the heart of the air cooling system. It’s the one that’s pumping the gas that is going to turn into liquid.

Without a working compressor your ventilation system will still work, but you will never get cool cold air.

Being an active component that is mechanically connected by the engine with a belt, it wants to work all the time. This will increase car fuel consumption.

Therefore, it’s a good idea to turn it on every couple of weeks to keep it in a good state. Don’t always think about the money spent on fuel because buying a new compressor will cost you more than this.

3. Faulty AC compressor clutch relay

car ac clutch relay

Like we said above, the compressor is connected to the engine via a belt. A clutch is used to engage or disengage the operation of the compressor.

The clutch is electrically powered via a relay. A relay is closing or opening the electrical circuit. Therefore, if the relay it’s not working properly, the electrical circuit it’s not closed and there is no current to engage the clutch.

The relay it’s based on a mechanical circuit so you can slowly knock it to see if the clutch it’s starting to engage.

This might be an easy and inexpensive fix of the AC blowing warm air in the car.

4. Broken condenser

broken car ac condenser

The condenser is the first component that is cooling the gas and as an effect this one takes liquid form. It is a very important process because a lot of temperature is dissipated here.

A broken condenser means that the gas might be still turned into liquid, although the temperature is higher than the desired one. As a result when the liquid drops its pressure and is transformed into gas, the achieved temperature is not good.

Inside the cabin you will feel cooler air, but not as cold as you desire.

5. Bad cooling fan

Although the condenser it’s made of materials that dissipate heat, a fan is needed to further help the process. The position of the fan, it’s right in front of the car.

As a result, is more vulnerable to the outside environment. During driving, fan blades can get broken or even the fan itself can get damaged.

Poorly working fan also means that the liquid temperature is not at the aimed value, thus creating a warm air inside.

6. Leaks or blockages along the refrigerator path

The refrigerator delivery path comprises multiple pipes, hoses and valves. Any leaks in these components will cause a lower pressure than wanted.

We want a lower pressure only to transform the liquid in gas, not on the high-pressure part of the system.

The same effect can have a blockage. It will cause components to work harder to achieve correct values. A test on the manifold gauges can help in diagnosing if the problem is in the delivery system. 

7. Worn out thermal expansion valve

worn out car thermal expansion valve

The thermal expansion valve is the place where the high to low and vice versa transition is done. This should be able to drop the pressure almost 10 times so that gas at a cold temperature is formed.

At the same time is also responsible to deliver the gas back into the compressor so that it can repeat the whole cycle.

In conclusion, it means that is a very important part of the whole system. Without this being fully functional, it does not cool air at a low temperature.

8. Blown clutch fuse

This one is also a part of the clutch’s electrical system. To protect the components of over current, they placed a fuse.

The fuse is placed in series with the other components in an electrical circuit so it can open the circuit in case of an issue.

Even if it’s an easy fix to replace the fuse, check the electrical components. A defect can be hidden, and as an effect, the fuse is blowing.

Check the car electrical schematics to find the location of the fuse and test it if you have the proper knowledge.

9. Electrical wires issues

The electrical layout of a car is very complex and very difficult to diagnose if you don’t have the correct expertise.

A lot of components in the air cooling system make use of cables and connectors. For this reason, it uses good connections, and proper quality wires.

Generally, you can do a visual inspection of everything. Firstly, all the connectors should be perfectly locked and are not loosened. Secondly, check for obvious damages to the cables. 

You can easily observe if a wire has cuts or is melted. Fix it well so later you will not have additional problems.

10. Broken compressor clutch

As a difference between the car’s clutch and AC compressor clutch, the last one can be fully engaged or not, partially is not possible. Thus, a slipping clutch is a problematic component.

A coil is inducing a magnetic field that is causing the clutch plate to connect to the compressor shaft. In effect you can say that if something is wrong in this assembly, the clutch will not engage and thus the compressor will not function.

Try to fix it first, before buying a new AC compressor which can be expensive.

How can you fix an AC that blows warm air?

While there are multiple causes of AC that is blowing warm air, the fixes can follow a logical order:

  • firstly check if you have a good system pressure – if the pressure is too low you need to add refrigerator in the system;
  • secondly you can do a visual inspection of all the pipes/hoses/interconnections of the AC system. When the refrigerant is leaking it is difficult to see it because it becomes a gas, but a check of the delivery system can always help;
  • thirdly check if there are any issues with the condenser/cooling fan placed in front of the car – if there is debris or suspect things covering it try to clean them;
  • next step is to check if the compressor is engaging properly – press the AC ON button while you can check the sound like a “tick” when it’s working
  • be sure that every connector that is related to the AC system is properly inserted and does not have any loose contact;
  • exchange the AC electronic control unit (ECU) with a new one if not any of the above steps is working;


A car AC system is a very important part that offers comfort for drivers and passengers. A broken AC system that blows warm air in the car is a problem that needs to be solved for the satisfaction of cold air. Sometimes might be easy and inexpensive, but sometimes it might be costly.

It’s up to you how you maintain the system and how much care you have!




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