Why Is Fog Coming Out Of AC Vent In Car?

A car’s AC vent is vital for a comfortable user experience while on the road. It regulates the interior temperature of the cabin and helps create a pleasant environment. 

Therefore, it’s frustrating to notice the unit producing fog suddenly.

The probable reasons behind fog coming out of the AC vent in a car are given below;

  • Side-effect of condensation
  • Heater box overflowing
  • Defective compressor clutch
  • The odor from burnt wires

Our article will help you understand the nature of the fog, the reason for its emission, and the cause of AC vent smoking. Furthermore, it will also provide practical solutions to help you combat the problem.

Why Is Fog Coming Out Of AC Vent In Car
Why Is Fog Coming Out Of AC Vent In Car

Characteristics of the Fog

Fog emission from the AC vent in cars typically occurs when its cold air comes in contact with the warmth outside. Water vapor forms into water droplets and creates fog if the internal temperature is not on par with the dew point.

AC vents also emit fog if there is a lack of proper maintenance. If the fan isn’t rotating, fast enough or dirt has filled the air filter, the low circulation causes fog to form. 

It will continue to come out as long as the cabin’s humidity is high.

Although the car’s health risks are low if fog comes out of the AC vents, people often confuse it with smoke. The smoke’s steam and odor indicate something is wrong with the car.

A car’s air conditioning unit requires an evaporator core and a heater core to function correctly. Small leaks often form on the heater core’s body due to corrosion. A clear indicator of this problem is if steam starts to come out of the AC vents.

The coolant leak on the passenger footwell and its smell from the steam are clear indicators of pinhole leaks on the heater core. Furthermore, leakage in the evaporator core can cause refrigerant sprays from the AC vent.

In such a scenario, you must take immediate action to repair the holes in the components. 

Coolant is a highly explosive substance and will increase the risk of fire hazards in the cabin if it leaks. Therefore, it can lead to severe injuries.

Causes of Fog Coming from AC Vents

Below are the most common reasons why a car’s AC unit may break down and cause fog to come out of the vents. Keep on reading to have a deeper understanding of the causes to apply fixes quickly.

1. Byproduct of Condensation

Cars tend to have a musty and warm interior when you enter for the first time. Therefore, the air conditioning unit needs to be operational for a while before the cabin starts to feel cool and comfortable. 

In this scenario, it’s natural to notice fog come out of the vents when you initially turn on the AC. 

The cold air from the unit comes into contact with the cabin’s warm atmosphere and causes condensation. Thus, the moisture forms into droplets.

The condensation of moisture into droplets often creates the image of mist coming out of the AC vents. There are no issues with the system in such a case. You will notice the fog emission stops once the internal temperature becomes cooler.

2. Overflowing Heater Box

AC systems in cars tend to have heater boxes that circulate coolant to produce cold air through a fan. Sometimes, excess water accumulates in this component due to clogging its drain hole. As a result, fog starts to come out of the vents.

An easy way to identify if an overflowing heater box is causing the problem is by checking the fog quality. 

White mist with no odor is a clear sign of this issue. Flooding in the component causes the steam within to condense and produce smoke when combined with cold air.  

3. Air Filter Clogging

Out of the many components of a car’s AC unit, air filters play a vital role. It regulates airflow in the cabin by ensuring a fresh intake constantly while expelling the stale ones. 

Continuous use can lead to the build-up of dust on it.

Too much dust on the air filter leads to clogs in the component. Thus, fog starts to come out of the AC vents. To avoid this scenario, wash the part regularly and conduct routine maintenance to check its quality.

4. Defective Compressor Clutch

Sometimes, issues in different car parts often cause smoke from them to enter the AC. 

The compressor clutch is the most significant component in this scenario. It starts to produce excessive smoke, which takes the form of fog when exiting the vent.

Fog from the compressor clutch can be detrimental to the passenger’s health. Various health hazards like lung inflammation and difficulty breathing can occur if people inhale it. Therefore, it’s strongly advised not to drive under such circumstances. 

5. Burning Wires

Among the many reasons for fog coming out of the AC vents, this one is probably the most hazardous. This problem is common in older models. Years of rigorous use or accidents can cause the wires to burn.  

Damaged wires can lead to a plethora of electrical issues for the car. 

Water leakage also occurs alongside fog emission, and a horrible odor fills the cabin in this case. Immediate repairs are necessary to ensure the vehicle’s performance remains optimal.

Related Post: Air Only Blows Through Defrost Vents | Causes & Solutions

How to Stop Fog Emission from AC Vents

Below are three of the most reliable solutions to stop fog from coming out of a car’s AC vent. Following them should help you easily fix this problem.

1. Repairing Faulty Compressors

If clutch troubles are the reason behind fog emission from the AC vents, then you will have to replace the compressor. 

You can also consider repairing the component if you have good experience with automobile maintenance.

However, it’s tricky to repair the compressor due to its sensitive and intricate structure. Therefore, it’s best to completely replace it to avoid any complications or problems in the future. 

2. Change the Coolant

Coolant quality can drop drastically due to leakage. Therefore, it’s necessary to fill the car with fresh coolant. 

Mixing new fluid with the older one can have disastrous results on the AC unit. So, make sure the container is empty before adding it.

If you want to avoid the hassle of adding coolant to the car, then it’s best always to keep an eye on the coolant level. Maintaining the core’s condition will keep the vehicle operational on the current fluid for a significantly long time.

3. Conduct Routine Maintenance

Regular maintenance is necessary to keep an eye on the car’s AC and surrounding components. 

You will be able to identify any issues the parts may face through it quickly. These issues include drain blockages and clogged filters.

If you notice any of these two problems with your car, you should consult a mechanic immediately. 

Fog emission will stop as soon as the components are back in proper condition and cool air will flow normally through the vents again. 

Related post: Should the Radiator Fan Turn on When AC is on?

Frequently Asked Questions [FAQs]

a. Do car AC naturally produce white fog?

A car’s AC typically produces white fog if the cabin’s temperature is under the dew point. Otherwise, any other cases occur due to inefficiencies in the components. 

b. Why are my car’s AC vents sweating?

AC vents in cars start to sweat due to condensation. Another byproduct of this process is fog coming out of the system.

c. Can condensation from the AC damage the car?

The fog created from condensation in the AC vents can cause water accumulation on the windows and break their seal. Furthermore, it can cause rusting in the car’s interior mental.


Although fog from a car’s AC vent creates no immediate danger to the car, it can damage it if it’s left unrepaired for too long. 

Since the diagnosis process is relatively simple, it’s best to conduct one immediately to quickly identify the solution. 

Stopping the emission of fog from the AC vent will not only help cool the car faster but also help you drive safely due to clear vision. Moreover, the internal environment of the cabin will also be cozy and comfortable.

Related Post: Car Sputters When Air Conditioner Is On

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