What Causes a Rotten Egg Smell in a Car?


Sep 09th, 2021

What Causes a Rotten Egg Smell in a Car?

Cars give off all kinds of smells, but not all of them are harmless. One of the most common signs of a problem is when you sense a rotten egg smell in your car, as it could point to several different issues.

A rotten egg smell in your car suggests a problem with the catalytic converter, fuel pressure sensor, leaking battery acid, or old and leaking transmission fluid. That smell is hydrogen sulphide, also known as sulphur. The gas can harm your health, so you should aerate the car and drive it to your mechanic immediately.

In the following sections, you'll learn everything you need to know about the smell of rotten eggs in your car. We will dive deeper into the possible causes and how you can go about solving the issue.

Let’s get started.

Understanding the Rotten Egg Smell in Your Car

Let’s talk about why your petrol or diesel car smells like rotten eggs in the first place, despite the absence of eggs anywhere in your vehicle. 

To start with, you're not inhaling the smell of real eggs. Instead, what you're breathing in is hydrogen sulphide, commonly referred to as sulphur, which is a colourless gas. When it hits our nose, we immediately identify it as the same smell as rotten eggs.

So, if it wasn’t clear enough before, we’re not dealing with actual eggs in your car but, instead, a gas that smells like it.

Now, the big question is, what’s producing that smell? In a car, that smell can come from several different components that aren’t working as they should.

Let’s explore each of them in the next section.

What Do I Do If I Smell Rotten Eggs in My Car?

Here are the most likely causes behind the smell of rotten eggs in your car. As we explore each reason, we’ll also look at the affected part and what you can do to resolve it.

Catalytic Converter Issue

What a Catalytic Converter Does: A catalytic converter (aka ‘cat’) is a component that sits somewhere along your car's exhaust system. As combustion takes place inside your car’s engine, the ‘cat’ converts harmful emissions into steam and other safer by-products before they are released out of the exhaust pipes and into the atmosphere.

What Happens When the Catalytic Converter Fails: There are tell-tale signs that the catalytic converter is failing, and these are: 

  • the car smells like rotten eggs, and 
  • the check engine light is on

Typically, the catalytic converter will convert hydrogen sulphide to sulphur dioxide, which is harmless. The problem arises when the converter is clogged or broken and thus fails to convert hydrogen sulphide, which smells like rotten egg and can escape into the cabin. 

How to Fix the Issue: The catalytic converter is a critical component of your car. So, if there’s anything wrong with it, the best and safest solution is to replace it entirely. You could attempt to remove blockages using a catalytic converter cleaner, but it might not be effective in resolving the overall issue.

Failing Fuel Pressure Sensor

What a Fuel Pressure Sensor Does: The fuel pressure sensor is responsible for controlling how much fuel your vehicle uses. Based on the sensor’s readings, the car’s system will adjust the fuel volume and timing as it makes its way through the system and into the engine.

What Happens When the Fuel Pressure Sensor Fails: When the fuel pressure sensor fails, it can result in the engine receiving too much fuel to process and unburnt fuel reaching and overwhelming the catalytic converter.

When the catalytic converter is overwhelmed, it cannot convert the exhaust into safer gases, and hydrogen sulphide will make its way out of your exhaust pipe, smelling of rotten eggs.

How to Fix the Issue: The fuel pressure sensor is another critical part of your car’s combustion system. So, if it fails, your best bet is to replace it entirely to prevent a wide range of engine problems.

Leaking Battery Acid

What a Car Battery Acid Does: The car battery is located under the hood and contains battery acid, which converts the stored chemical energy into electrical energy. Your car needs electricity to start and function correctly.

What Happens When Acid Leaks from a Car Battery: A rotten egg smell from your car’s battery is a clear sign that battery acid is leaking out from the unit. You may also notice corroded terminal caps or bubbly liquid oozing from a vent cap. The battery cell may rupture, or worse, the battery can explode. 

How to Fix the Issue: As you might expect, the fix is to replace the car battery entirely. Even if you find that the battery is still working, an immediate replacement is necessary. Not only will the battery fail very soon, but that leaking acid will damage any other car part it comes into contact.

Stale Transmission Fluid

What Transmission Fluids Do: Your car’s transmission fluid is designed to lubricate the movements of your transmission system, i.e., the moving gears inside. 

How Can Transmission Fluid Fail: Like any other fluid in your vehicle, the transmission fluid must be changed as part of your scheduled routine maintenance. When you leave the fluid in the transmission for too long, it will not stay contained and will leak towards other vehicle parts.

The smell of rotten eggs is likely coming from that old transmission fluid that’s moving about.

How to Fix the Issue: Have your mechanic drain and replace the old transmission fluid to fix this issue. You’ll also want them to inspect the transmission system to ensure that any leaks are resolved as well.

To prevent this issue from happening again, make sure that your transmission fluid is changed out regularly. The general recommendation is to change it every 12 months or 12,000 kilometres. The more frequent you drive your vehicle, the shorter this interval becomes.

Is a Rotten Egg Smell in Car Dangerous?

Yes, a rotten egg smell in your car can be dangerous. As mentioned before, the smell comes from the hydrogen sulphide or sulphur that you inhale. That gas can cause harmful effects to your body, so it's not something you want to tolerate at all.

If you notice the smell of rotten eggs while driving your car, the first thing you’ll want to do is roll down the windows. Then, maximise the ventilation in your car’s cabin to ensure that the gas flows out of the car and you have plenty of fresh air to breathe.

Then, drive your car to your trusted mechanic immediately. The smell will not go away until you solve the root cause. 

To learn more about similar issues affecting your car, check out the blog Carpart.com.au. For your car part needs, please don’t hesitate to send us a request so that we can help you search for parts and reputable sellers. Just fill out this online form and wait for quotes – that easy!

By Ray Hasbollah