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Home / Symptoms / Rotten Egg Smell

Four common causes for a rotten egg smell from the vehicle and their related parts.

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Why Does My Car Smells Like Rotten Eggs?

A rotten egg smell in cars comes from a gas known as sulfur dioxide. The catalytic converter, fuel pressure regulator, fuel filter, etc., work with this gas; if any of these parts fail, they’ll make this gas smelly, which reminds of rotten eggs.

In short, the car's emission system makes sulfur dioxide odorless. Roughly speaking, the catalytic converter transforms toxic gases and pollutants from the engine combustion process into safer-to-breathe gases. However, when the catalytic converter has an issue, these gases go around in their full form, producing their natural bad smells and even causing allergies.

Watch out: Toxic combustion chemicals can harm humans and damage the car's catalytic converter, which controls emissions. So, even if the check engine light didn't pop but you notice a rotten egg smell, hurry up and take your car for an inspection.

Be aware if a car smells like rotten eggs. Here are some of the main risks:

Emissions problems: The rotten egg smell commonly comes from a faulty catalytic converter, which is part of the emissions system. A problem in this system means your car will release more toxic fumes and harmful gases, contributing to pollution.
Safety Issues: If the rotten egg smell results from emission issues, you and your passengers might be at risk of breathing harmful gases; in some cases, this can lead to fatalities. But, if the rotten egg smell comes from the battery, your car might suffer electrical issues and even leave you stranded randomly.
Health Concerns: The rotten egg smell isn’t supposed to happen in your car, if they do, this means that gases that should be are not being processed. These gases can be highly toxic and cause breathing problems, triggering allergies, or even contaminating and leading to fatalities.

Let's Get To The Bottom Of It!

Is your battery faulty or leaking?

Failed or damaged battery cases can cause sulfuric acid fluid leaks. Then, batteries with internal failures or overcharging swells create gases that stink like a rotten egg.

Is your car leaving leaks in the garage?

Leaking gear oil from differential fluid has the same problems and unpleasant odor as old transmission fluid leaking from other systems like transfer cases or manual transmissions; this might be the case.

Have you recently changed the oil?

As the engine burns gasoline, oil, and transmission fluid, a specific odor generates while the catalytic converter turns the exhaust stream into harmless gases. In this process, gasoline quality or engine oil overflow are common culprits of bad smells.

Have you noticed an increase in fuel consumption?

A fuel pressure regulator might fail. The pressure sensor regulates the engine's fuel flow. So, sensors reporting the wrong pressure to the engine control computer produce poor emission control. Also, a lousy sensor in a hot engine bay can result in leaks from faulty parts. A fuel filter can clog up and stop over time, resulting in low pressure. A worn-out filter fails similarly to fuel pressure sensors. A faulty fuel pressure sensor affecting the system's operation interferes directly with exhaust gases. A failing fuel pressure sensor can increase consumption, risking car fire. Typically, code scans can provide a complete snapshot of the vehicle, reporting these types of problems.

Did you recently fill the fuel tank?

Your car's engine having too much fuel might damage the catalytic converter, making the engine emission system malfunction from the fuel overfill and incomplete combustion, resulting in poor fuel mileage. The engine might be burning transmission fluid, impacting your car's catalytic converter. The catalytic converter neutralizes the emission of bad gases, like sulfur gas. But, a faulty catalytic converter won't work properly, setting toxic chemicals free.

The most common causes for a car smelling like rotten eggs

Here are the most common issues causing your car to make a rotten egg odor:

Engine oil leak: An engine oil leak can cause a rotten egg smell in a car when the leaking oil comes into contact with the hot surfaces of the engine or exhaust system. As the oil burns off due to the heat, it can produce a sulfurous odor similar to rotten eggs.
Fuel regulator leak: A fuel regulator leak can cause a rotten egg smell in a car due to the presence of sulfur compounds in gasoline. The fuel regulator is responsible for maintaining the proper pressure of the fuel system. If the regulator is leaking, it can allow small amounts of fuel to escape, which can then vaporize and cause the smell.
Differential/transmission fluid leak: A differential or transmission fluid leak can cause a rotten egg smell in a car due to the breakdown of the fluid. Both differential and transmission fluids contain sulfur-based additives that help reduce friction and wear. If there is a leak in either system and the fluid comes into contact with hot components, it'll cause a rotten egg smell.
Faulty battery: A faulty battery can cause a rotten egg smell in a car due to the release of hydrogen sulfide gas. When a battery is overcharged or damaged, it can produce hydrogen gas as a byproduct of the chemical reactions inside the battery. This might also cause the smell to come from the air conditioning system, affecting the cabin air filter.
Bad air-fuel mixture: A bad air-fuel mixture can cause a rotten egg smell in a car due to incomplete combustion. When the air-fuel mixture in the engine is not properly balanced, it can lead to inefficient combustion, resulting in the production of hydrogen sulfide gas. This gas has a characteristic rotten egg smell and can be emitted from the exhaust system when the engine is running.

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Frequently Asked Questions

Is your battery faulty or leaking?

Failed or damaged battery cases can cause sulfuric acid fluid leaks. Then, batteries with internal failures or overcharging swells create gases that stink like a rotten egg.
Is your car leaving leaks in the garage?

Leaking gear oil from differential fluid has the same problems and unpleasant odor as old transmission fluid leaking from other systems like transfer cases or manual transmissions; this might be the case.
Have you recently changed the oil?

As the engine burns gasoline, oil, and transmission fluid, a specific odor generates while the catalytic converter turns the exhaust stream into harmless gases. In this process, gasoline quality or engine oil overflow are common culprits of bad smells.
Have you noticed an increase in fuel consumption?

A fuel pressure regulator might fail. The pressure sensor regulates the engine's fuel flow. So, sensors reporting the wrong pressure to the engine control computer produce poor emission control. Also, a lousy sensor in a hot engine bay can result in leaks from faulty parts. A fuel filter can clog up and stop over time, resulting in low pressure. A worn-out filter fails similarly to fuel pressure sensors. A faulty fuel pressure sensor affecting the system's operation interferes directly with exhaust gases. A failing fuel pressure sensor can increase consumption, risking car fire. Typically, code scans can provide a complete snapshot of the vehicle, reporting these types of problems.
Did you recently fill the fuel tank?

Your car's engine having too much fuel might damage the catalytic converter, making the engine emission system malfunction from the fuel overfill and incomplete combustion, resulting in poor fuel mileage. The engine might be burning transmission fluid, impacting your car's catalytic converter. The catalytic converter neutralizes the emission of bad gases, like sulfur gas. But, a faulty catalytic converter won't work properly, setting toxic chemicals free.

Typical fixes to address the cause(s)

These are typical processes and repair services that fix a check engine light issue

The most common cause: Faulty battery

The most common reason for a car making a rotten egg smell is because of a faulty battery. Batteries have a specific chemical composition that can cause a rotten egg smell. In most cases, a chemical reaction between sulfuric acid, lead dioxide, and lead plates happens inside the battery, causing the odor. Still, this chemical reaction is mostly concerning for old and worn batteries.

Battery failures mostly happen when the vehicle has been running with the same battery for a long time.

Origin of the smell: Under the hood.
Symptoms it causes: Apart from the rotten egg smell, you might notice electrical failures, struggles to start, dimming headlights, etc.
High risk of priority: The battery can fail and break down anytime, leaving you stranded randomly.

How exactly does a battery fail and cause a rotten egg smell?

Batteries can contaminate. This type of contamination happens when the sulfuric acid inside the battery breaks, producing hydrogen sulfide gas. This issue typically develops in batteries that are repeatedly charged and discharged throughout their life cycle. So, old and worn batteries.

However, depending on manufacturing quality, new batteries can also develop similar problems.

Go safe: Ask for a mechanic to inspect your vehicle to track the rotten egg smell’s location and see if it means further damage. Many shops do transparent vehicle inspections and lay out your options after investigating your vehicle’s conditions.

What’s a vehicle inspection? It’s “detective work” on your vehicle, checking its systems to find if anything is preventing the car from working as expected. In this case, mechanics examine different parts of your vehicle to track where the rotten eggl smell is coming from and what’s causing it. They’ll also check the condition of components during their inspection, noting which are causing the problem and which got affected by it, recommending to replace them in order of priority.

Transparency example in a rotten egg smell issue service

Let’s see how a vehicle inspection defines the car service needed to fix a rotten egg smell issue in a vehicle.

In this scheduled service, the customer stated their car smelled like rotten eggs, especially after starting and driving, and the smell seemed to come from under the hood. The technician lifted the car’s hood and checked around.

While inspecting under the vehicle’s hood, the expert found a swollen and wet battery. By examining closer, there was corrosion around the battery’s terminals and a stronger rotten egg smell.

With these findings, the technician will look at the surrounding components to see if the battery leak contaminated other components. Then, they'll reinstall a new battery and check the electrical system’s condition.

The image below shows the technician’s first look at the bulging and corroded battery.

Battery internally shorted because of leaks and seal issues

In this case, the mechanic put the “Immediate Action” tag on the battery. They also recommended cleaning the battery tray and surrounding components.

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