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rotten egg smell in car air conditioner

Rotten Egg Smell In My Car Air Conditioner [5 Causes & Solutions]

When you start your car ac and notice it blowing out air with a rotten egg smell, you shouldn’t leave it unattended because it could indicate a problem with your vehicle. 

Simply put, a rotten egg odor in your car ac could be the result of a faulty catalytic converter, a broken fuel compressor sensor, a leaking car battery, an old transmission fluid, or decaying organic matter.

I will walk you through why these things can cause a smell of rotten eggs in your car ac and illustrate how to fix and prevent it from happening again. 

Why does my car air conditioner smell like rotten eggs?

The rotten egg smell is caused by a specific chemical called sulfur. Your car ac will smell like rotten eggs as long as something in your car can emit a gas containing this chemical.

1. Organic matter

Organic matter decaying can produce hydrogen sulfide gas, which has a rotten egg odor. As a result, the grocery you left in the cabin could be the culprit.

2. Malfunctioning catalytic converter

catalytic converter

A catalytic converter is found at the bottom of a vehicle and is responsible for converting harmful hydrogen sulfide into harmless chemicals, thereby reducing the environmental impact.

Normally, cold starting can cause the vehicle to emit a small amount of hydrogen sulfide because the catalytic converter cannot work effectively and transfer the hydrogen sulfide sufficiently at low temperatures. But this unpleasant odor will disappear shortly.

If the smell of rotten eggs persists in the air, it could indicate that the converter is broken and no longer capable of converting hydrogen sulfide. The hydrogen sulfide will enter your car through the air vents, and the ac will blow it out when it is turned on.

3. Broken fuel pressure sensor

The faulty fuel pressure sensor can hinder the work of the catalytic converter and lead to the emission of a rotten egg smell. 

This is due to the fact that the fuel pressure sensor is in charge of where the fuel goes. When it fails, a large amount of fuel will accumulate around the converter, causing a part of the hydrogen sulfide to get out of your car and then enter your car cabin. 

4. Old transmission fluid

The transmission fluid needs to be replaced on a regular basis. If not, it can leak out, and the stagnant fluid can release a rotten egg smell, which will eventually get into the air conditioning system in your vehicle.

5. Leaking car battery

Believe it or not, a leaking car battery can release an unpleasant odor. The battery is composed of water and sulfuric acid, and the sulfuric acid can be converted to H2S when it is heated up.

Due to extreme weather or other reasons, the seal of the battery can be damaged, and the acid will start to transform and give off a foul smell. 

Finally, the rotten egg smell will enter the ac system, leading you to believe that something is wrong with your car ac.

How to get the rotten egg smell out of your car air conditioner?

Note that you should stop driving your vehicle if you notice an egg smell from your car ac. Sometimes, it can lead to serious safety problems.

Remove all the decaying material

If your car is new or recently serviced, the rotten egg smell in the car ac may not be caused by car parts.

Give your car a thorough inspection and dispose of any decaying food, such as vegetables.

Visit a certified mechanic

see a mechanic

If the odor is not caused by organic matter, you should consult a mechanic to determine the source of the problem.

You may need to replace a converter or a fuel pressure sensor, which you should not attempt to do on your own.

How do you prevent the rotten egg smell in your car ac from returning?

It will be very frustrating when you see the awful rotten egg smell coming back. To avoid it, you should take some measures.

Never leave food in your car

People, especially mischievous children, enjoy eating food in the car. It is acceptable to do so, but please remember to clean up any trash and remove any food before leaving the car.

After a night, the food in the car can emit a foul odor, ruining your good mood in the morning.

See a mechanic regularly 

As you are aware, repairs can be quite costly. However, if you keep your vehicle in good condition at all times, it will be less likely to break down.

A leaking battery, for example, can endanger several parts of your vehicle. If the mechanic detects the problem early on, you can simply replace the battery without having to clean other parts.

Furthermore, because the transmission fluid is replaced on time, it will not clump and emit an odor.


The rotten egg smell in your car air conditioner is not only disgusting but also a sign of something wrong with your car itself. 

The broken catalytic converter or fuel pressure sensor is to blame, aside from the decaying food. Some people are unaware that a leaking car battery or transmission fluid could be the source of the odor.

If you encounter this problem, you should consult a mechanic because some situations may be hazardous.

If your air conditioner in the home also has the same problem, please read my post about why an air conditioner smells like rotten eggs here.

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Liz Yang is the founder of Airsmartly. She has been working at home for a few years and realizes that the performance of the HVAC system plays such an important role in our life. She has tested more than 150 products in person, including humidifiers, air purifiers, dehumidifiers, and ACs, and wants to share tips about using or troubleshooting these products with you. Her uncle is an HVAC expert with over 30 years of experience in the field, and often offers assistance when she is unsure how to handle a situation. He is also in charge of reviewing the articles on this site.

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