Air Only Blows Through Defrost Vents | Causes & Solutions

Have you ever felt this where you turn the AC on expecting cool air, but you’re hit with a warm breeze coming out of the defrost vents? Such problems can be irritating, but fortunately, they can be fixed easily. 

But what causes this? Air only blows through defrost vents due to the following reasons:

  • Faulty electrical components
  • Electrical issue
  • Vacuum leak 

Here, we’ll go over these problems and discuss which components may be at risk. Furthermore, you’ll be able to troubleshoot your vehicle and take the necessary steps to fix it once you’re done with this article. 

Air Only Blows Through Defrost Vents Causes & Solutions

What Causes Air to Blow through Defrost Vents Only?

The issues that cause air to blow through the defrost vents only are as follows:

Malfunctioning Mode Door Actuator

The mode door actuator plays the crucial role of determining which vents the air will flow out of in a car’s air conditioning system. However, these are electronic components that can wear out over time and become dysfunctional. 

It usually comes with strips and components made of plastic that can get damaged over time. Apart from that, excessive load, inappropriate wiring, etc. can lead to a malfunction. 

Another way of knowing if the mode is broken or not is, you need to listen for a clicking noise when the AC is on. If there is one, then you can be certain that there’s an issue with the mode. 

Faulty Blend Door Actuator

The blend door actuator’s main purpose is to allow or block the air passing through the heating core. This way, it can regulate the temperature of your vehicle’s system. 

However, this component is usually made of plastic and can wear out after years of use. Also, if the electric motor burns out for some reason, this component can stop working altogether. 

When this component is damaged, you’re likely to hear a ticking sound from the dash. 

Blend door actuators can be replaced at a very reasonable price. So, if you’re facing this issue, you can buy an aftermarket actuator and replace it yourself, or consult a professional. 

Blower Motor Damaged

The blower motor works with the blower resistor to control the flow of air that’s being pushed through the vents. If the blower motor is damaged, it fails to push any air down the vents, so you may get air blowing through the defrost vents only. 

The Blower motor is usually damaged from years of wear and tear. Usually, these get damaged when they get overheated due to airflow restrictions. In other cases, they can get rusty, which can make them brittle. 

You can detect if the blower motor is damaged or not through two symptoms. Firstly, look out for a vibrating or screeching noise. Secondly, you may smell something burning if the blower motor is overheating. 

If the blower motor is just rusty but the components are working fine, you can get it cleaned of rust professionally. You can also attempt to clean it at home with a rust dissolver. 

However, if the component itself is broken, then you’ll have to replace it or replace the affected part of the component. 

Faulty Blower Resistor

As discussed, the blower resistor and blower motor work in cohesion. The function of the blower resistor is to push the air through the dashboard vents into the vehicle. 

Also, the resistor helps determine the amount of airflow into the system. Simply put, it lets you select different fan speeds for the AC. 

However, certain issues can lead to a blower resistor failure. It’s commonly damaged due to thermal overload. You can check whether the blower resistor is broken by checking the fan settings. 

If the airflow doesn’t change regardless of how much you change the fan settings, you’ll need to replace the blower resistor. 

Bad Relay

The electrical relay helps continue the entire ventilation system of your vehicle. If some part of the electrical relay gets damaged, then the system will fail to ensure optimum airflow. 

This is also when the air coming into the vehicle will have sufficient momentum to flow by itself, as no external factor will control where the air will flow through. 

So, depending on the structure of the ventilation system, the air may blow only through the defrost vents. 

This can be the most complicated issue to fix, as the electrical wiring can go wrong at many different places. We highly recommend visiting a professional for getting this sort of problem fixed. 

Blown Fuse

The purpose of the fuse is to regulate the voltage in the ventilation system so different electric component is supplied with the appropriate amount of power. 

However, the fuse might blow under pressure, which will halt the ventilation system. 

When the ventilation system stops working, the air might force itself through the defrost vents only. 

In this case, you’ll need to check the fan breaker, ventilation breaker, and clutch breaker. If any or multiple of these components are damaged, then you’ll need to fix these to restore your vehicle’s ventilation system to proper. 

Vacuum Leak

A lot of vehicles made between 1990 to 2005 come equipped with vacuum-operated vents. In this system, there’s usually a vacuum tank that’s connected to the vent doors with hoses. 

Vacuum Leaks
Vacuum Leaks

By default, the doors are set to the defrost position. When the user makes any changes to the vent knob, he is changing the position of the vents as necessary. 

However, if any of the vacuum lines break, the entire system collapses as there’s no vacuum holding the vents in position. Thus, all the vents return to their default position, which is the defrost vent. 

This leads to air blowing in through defrost vents only. To fix this, one can either get service from a professional, which will cost a fair bit. 

If you want to fix it yourself, you’ll need to detect the vacuum leak first. 

You can use a smoke machine to detect if any of the hoses are leaking smoke or not. Once you determine which line is leaking the smoke, you can get it replaced to solve the problem. 

Clogged Air Intake 

In certain cases, the air ducts inside the ventilation system can get clogged with dirt and grime. This happens if you use the ventilation regularly for a long time. However, it mainly depends on the amount of dust particles in your environment. 

Regardless, it’s an issue that can lead to major obstructions in the airflow system of your vehicle. It can close off multiple ducts altogether, forcing all the air to go out of the defrost vents. 

To fix this problem, you need to open the air vents and clean them manually. We recommend getting professional help as you’ll need expertise and tools to clean the ventilation system properly. 

Related Post: Why Is Fog Coming Out Of AC Vent In Car?

Frequently Asked Questions [FAQs]

How Often Should You Clean Air Vents?

Dirt and grime accumulate over the air vents, so they should be cleaned at least once a month to keep the system running properly. 

Why Is My Car AC Blowing Hot Air?

Your vehicle’s AC might be blowing hot air because a component responsible for controlling the temperature is broken or failing. 

How to Improve Airflow of Car AC?

You can ensure better airflow out of your vehicle’s AC by carrying out regular HVAC maintenance. 


When air only blows through defrost vents, you need to get your vehicle checked for multiple issues. Now, multiple of the issues mentioned above can take place at once. So, you may not see results even after fixing one of them. 

So, once you start troubleshooting, we recommend checking your vehicle for all probable issues. This way, you’ll be able to restore your vehicle’s AC to its original state. 

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