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Your Car Shouldn’t Smell Like Rotten Eggs: Here’s What’s Wrong

Your Car Shouldn’t Smell Like Rotten Eggs: Here’s What’s Wrong

Have you ever had a car pull out in front of you that smells like rotten eggs? Or maybe your own car is emitting an eggy odor and you’re asking “Why does my car smell like rotten eggs?” This is really not something any of us ever hope for. Not only is the smell terrible, it probably means something is wrong.

While you may try to overlook it, understanding why your car smells like rotten eggs is important for keeping your engine running smoothly and safely.

Here, we’ll discuss the potential causes of this issue and what steps you can take to eliminate the stench.

Why Does My Car Smell Like Rotten Eggs?

A car smelling like rotten eggs is almost always because of a sulfur-based compound. It can be related to fuel, transmission fluid, or even the car’s battery. 

Below are some of the most common causes that your car might smell like rotten eggs. Let’s dive in and look at each possible cause and learn what you can do to fix the problem.

Pro Tip: Time for some RV maintenance? Make sure you know these 5 Red Flags to Watch for With an RV Repair Shop.

Man looking at vehicle on the side of the road
The smell of rotten eggs is never a good sign while cooking, or for your vehicle.

The Catalytic Converter 

Problem: The most common cause of the rotten egg smell in a car is the catalytic converter. A catalytic converter processes harmful chemicals caused by combustion, converting them into substances like water and steam.

The combustion process of gasoline fuel produces a lot of compounds, one of which is So2 or sulfur dioxide. Under proper operation the catalytic converter can oxidize these chemicals to something you can’t smell, however, under some normal circumstances, the catalytic converter will convert the So2 to H2S or smelly hydrogen sulfide. This happens naturally when the catalyst temp is too high, exhaust velocities are too low or the fuel mixture is too rich.

Thus if you smell an eggy smell when slowing down from high speeds with a hot engine it may be completely normal, however, if your car smells all the time or at idle then your catalytic converter is probably failing. Over time, these chemicals and gasoline can clog the catalytic converter, which hampers it from properly converting these chemicals into a less harmful exhaust. 

Replacing The Catalytic Converter 
Your catalytic converter may be causing your vehicle to smell like rotten eggs.

Aside from making your car smell like rotten eggs, a failing catalytic converter can cause problems with starting and accelerating, reduce gas mileage, and cause the check engine light to come on.

How to Get Rid of It: If there’s a contaminant in the catalytic converter and you catch it early enough, it’s possible that a fuel additive cleaner may be able to unclog it. You might also be able to remove and manually clean it. While we do not have experience with a product like this we have heard good reports of it improving catalytic converter performance at least for a while.

Dura Lube Severe Catalytic and Exhaust Treatment...
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If the problem continues or gets worse it’s important to get the catalytic converter replaced. Before you do so have any codes the car is throwing checked out as there are many engine problems that can cause a rotten egg smell that the catalytic converter cannot fix. For example, leaking fuel injectors, bad spark plugs, or clogged intakes can cause a rich fuel mixture that causes an incomplete burn. It’s important to figure out what the root cause of the failure was if the catalytic converter got clogged.

Why my car smells like rotten eggs

The Car Battery

Problem: Another possible leading cause of your car emitting a rotten egg smell is the battery. Car batteries contain sulfuric acid, which should remain within the battery casing. Sulfuric acid can be broken down into H2S (hydrogen sulfide) which creates a very strong rotten egg smell. A battery that has been overcharged or has an internal short is likely to emit this gas.

How to Get Rid of It: The safest way to eliminate the rotten egg smell caused by a damaged battery is to replace it. If you intend to replace the battery yourself, be sure to wear gloves and eyewear that protect against any exposure to sulfuric acid. If you’re uncomfortable dealing with caustic substances, have a professional replace the battery as soon as possible.

When replacing a battery that was smelling like rotten eggs its important to check the car charging system to make sure the car was not overcharging the battery. If it was it will do the same to the new battery and destroy it as well. Check the voltages while the car is operating and make sure they are falling within safe ranges, otherwise, the car’s alternator or regulator may be bad as well.

Fuel Mixture Running Too Rich

Problem: If an engine is dumping too much fuel into the combustion chamber it will not burn completely. Not only will this cause poor performance and fuel economy but will also cause contaminants to make it into the exhaust. This can be a cause of the catalytic failure we talked about above. There are many things that can cause an engine to run too rich but here are some of the most common.

A car’s fuel pressure sensor regulates fuel flow through the engine. When this sensor fails, it allows the fuel mixture to get too rich. This leads to clogging or damage in the catalytic converter, creating a rotten egg smell.

Likewise, bad spark plugs can cause the fuel not to burn completely or cause improper ignition. Stuck air intake valves or leaking injectors can all do the same thing and increase the fuel mixture.

How to Get Rid of It: If you are lucky and catch the failing part soon enough, you may be able to simply replace it. But the catalytic converter is likely already clogged when the rotten egg smell reaches your nose. 

When this is the case, you need to replace the part and either clean or replace the catalytic converter. If you are not capable of diagnosing a fuel mixture problem yourself you will need an experienced and trusted mechanic to look it over for you.

Woman repairing car engine
Regular maintenance can help prevent things from going awry in your vehicle.

The Fuel Filter

Problem: Like a failing fuel pressure sensor, a failing or worn-out fuel filter can cause your car to smell like rotten eggs. When the fuel filter gets clogged or worn out, it allows a fuel mixture that’s too lean, which can again lead to a poor performing catalytic converter.

How to Get Rid of It: You’ll need to replace the fuel filter. If you do this soon enough, that may be the extent of the fix. But more than likely, you may need to clean or replace the catalytic converter.

The Transmission Fluid

Problem: Leaking transmission fluid is another cause of the dreaded rotten egg smell. When transmission fluid gets old, it can cause seals to wear out, creating a leak in the system. If transmission fluid leaks onto a hot engine, it can result in that rotten egg smell.

How to Get Rid of It: When this happens, you’ll need a mechanic to track down the leak. This typically requires replacing seals or gaskets, flushing the transmission, and refilling it with fresh fluid.

Leaking Transmission Fluid
Leaking transmission fluid will cause your car to emit a rotten egg smell.

How to Prevent Your Car From Smelling Like Rotten Eggs

Regular maintenance is the best way to prevent your car from smelling like rotten eggs. Make sure to consistently check all fluid levels per the manufacturer’s recommendations. Keep them at required levels, replacing them as necessary.

This also applies to the various filters in the car. There are more filters than ever in modern cars, but they still need to be maintained and regularly replaced.

Many of us are good at hoping that any problems that pop up will disappear if we ignore them. This rarely holds true. Any time an error code or something with your car seems off, get it checked out immediately. The sooner you address an issue, the easier it is to fix it with less damage.

Pro Tip: Keep your truck in tip-top shape by using these tips on How to Remove Rust From Your Truck Undercarriage.

Car Maintenance: 10 Things Every Car Owner Should Know - The Short List

Is It Safe to Drive a Car That Smells Like Rotten Eggs?

If your car is emitting a putrid smell of rotten eggs, it’s essential to take the necessary steps to diagnose and fix the problem. Driving a car that smells like rotten eggs can be dangerous, as it may indicate damaged or malfunctioning components that can affect your vehicle’s proper functioning.

To ensure your safety and avoid further damage, take steps to fix it immediately or contact a qualified auto technician or mechanic who can properly inspect and repair your car as needed.

Have you ever experienced your car starting to smell like rotten eggs? Tell us what the problem was in the comments!

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About Mortons on the Move

Tom & Caitlin Morton of Mortons on the Move gave up the stationary life for one where they are constantly on the move. They are full-time travelers, television hosts, and digital media producers.
They left their jobs, sold their house and possessions, and hit the road in September 2015 in their full-time “home on wheels”. Since then they have traveled the US, Canada, and even internationally by RV.
Now, they are Discovery Channel & PBS TV Co-stars of The RVers, producers of “Go North” on Amazon Prime, co-founders and instructors of RV Masterclass, and contributing authors for and an Arizona travel guide.

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Sunday 19th of February 2023

I don't understand how a plugged up fuel filter can cause the engine to run too rich. If the filter is plugged up, it wouldn't allow enough fuel so it would run lean. An engine running too lean could cause the catalytic converter to malfunction which then may cause a funky odor.