Does Your Car Smell Like Rotten Eggs? Here's Why, and What to Do About It

There are many smells that can get into your car. Some of these come from the world outside, some come from items that you bring into the car, and some come from the mechanical operation of your vehicle. Some of these smells are pleasant, while others are not. When you have an unpleasant smell inside your car that won’t go away, it becomes a matter of your comfort and that of your passengers. If that smell indicates a problem with the way your car is running, it becomes a warning sign of a serious and potentially expensive issue that must be dealt with as soon as possible. The smell of rotten eggs is one of these.

There can be many reasons why your car smells like rotten eggs. Some of these are annoying but nothing to be concerned about, while others can be extremely serious. Fixing the problem can be somewhere between very simple and extremely complex. Once you understand why your car smells like rotten eggs and how to fix it, you can be back on the road.

Here’s our guide to the causes of why your car smells like rotten eggs and how to fix it.

What makes that rotten egg smell?

To explain how the smell of rotten eggs is created, let’s talk some basic chemistry. What most people would identify as the smell of rotten eggs is actually made by the compound hydrogen sulfide, identified by its chemical name H2S. It is made up of hydrogen and sulfur.

While pure sulfur by itself is tasteless and has no smell, it can become very stinky when combined with other elements. Skunks, for example, use sulfur compounds to produce their defensive odor response. Rotten eggs actually produce hydrogen sulfide as they decompose. And your car can also have a variety of problems that can be indicated by the smell of rotten eggs/hydrogen sulfide. Let’s start with the most common sources:

Actual rotten egg issues

Rotten eggs in your car: Before we get deep into the various automotive problems that can cause your car to smell like rotten eggs, it will be helpful to eliminate the most direct and obvious cause of the problem: What smells like rotten eggs is actually rotten eggs!

Solution: Check your vehicle thoroughly for the presence of rotten eggs. Even if you find no actual rotten eggs, continue to check your car for remnants of fast food and snacks that may have been left in the vehicle and are in the process of decomposing, causing those items to smell like rotten eggs. If this is the case, remove the eggs and/or other food items, dispose of them, and follow these handy interior smell-removing tips:

• Vacuum the carpets and the seats (get into all of the crevices)
• Rub baking soda into the carpets, let sit for a few hours, then vacuum it up
• Leave a large chunk of grilling charcoal inside the car for a few days to absorb odors
• Try covering up odors with vanilla extract, mint, or orange oil-soaked cotton balls, or a small bag of ground coffee
• If all else fails, have a professional vehicle reconditioning service thoroughly clean and deodorize your interior

Dead animal issues

A small animal died in your car: If your car is parked outdoors, especially in a rural area, small rodents like mice, rats, and squirrels may find that your car provides them with a safe, protected place to live and even raise a family. This is much more likely if your car is not driven every day. These rodents can climb into the vehicle’s under-hood area from beneath and nest inside the air circulation system. They also like to chew on the insulation found on your car’s electrical wiring. When one dies, either from natural causes or getting sucked into a ventilation fan, its body will decompose, giving off sulfur compounds that will smell like rotten eggs.

Solution: Your mechanic will need to dismantle your car’s air intake and ventilation system to the point where the animal’s body and all related debris can be removed. After this, the entire system will need to be cleaned, sanitized, and reassembled. This process is very labor-intensive, which usually means expensive. You may want to check with your car insurance company to see if you have coverage for rodent damage and whether it is worth making a claim.

Catalytic convertor issues

Bad catalytic convertor: Just about every gasoline-powered vehicle has a catalytic convertor as part of its exhaust system. Your catalytic convertor’s job is to chemically scrub your car’s exhaust of harmful pollutants before they can escape into the atmosphere. As the fuel combusts inside your engine’s cylinders, the sulfur in the fuel becomes hydrogen sulfide, or H2S. This is the same compound that produces that rotten egg smell. When the hydrogen sulfide flows through the catalytic convertor, the platinum in the convertor normally turns the smelly hydrogen sulfide into odorless sulfur dioxide (SO2). If your catalytic convertor is bad, it may be unable to do this, making your car smell like rotten eggs. You may also notice poor gas mileage, starting and acceleration problems, and the Check Engine Light on your instrument panel coming on.

Solution: Your mechanic can check your catalytic convertor to determine whether it is faulty and replace it if necessary. A new catalytic convertor should eliminate the rotten egg smell. Depending on your car’s age and mileage, replacement of the catalytic convertor may be covered under the vehicle’s emissions control system warranty. Check your owner’s manual for the details of your car’s emissions control warranty.

RepairPal Tip: Your catalytic convertor is a key part of your car’s emission control system. It must be in proper working order to prevent the release of unhealthful pollutants. A bad catalytic converter can cause your car to fail its smog test, requiring repairs before you can legally register it.

Fuel-related issues

Over-rich fuel condition: Your car’s engine is designed to operate within a specific fuel to air ratio range, which provides the best balance of performance, fuel economy, and low emissions. If your fuel system should have a problem and use too much gasoline relative to air, you will have what is called an over-rich fuel condition. This means that the excess fuel that remains unburned during the combustion process will produce much-higher-than-normal levels of that smelly hydrogen sulfide in the exhaust. When these high levels of H2S hit your catalytic convertor, some of it cannot be processed into odorless SO2, so it comes out of your tailpipe smelling like rotten eggs. You may notice poor drivability, bad fuel economy, and the illumination of your Check Engine Light.

Solution: Your mechanic can electronically scan the operation of your engine computer, diagnose the source of the over-rich condition, and repair it. An over-rich fuel condition can be caused by a bad fuel pressure sensor, a worn-out fuel filter, or a faulty fuel injection system. If you notice this problem, don’t delay having it fixed – if it goes on too long, it can damage your catalytic convertor.

Fuel with excessively high sulfur content: Sulfur is a naturally-occurring component of the crude oil that gasoline is refined from. In excess quantities, sulfur impairs the effectiveness of emissions control systems. As of 2017, the US EPA has limited the sulfur content of gasoline to a maximum of 10 parts per million. Occasional refining errors or other issues may result in gasoline that exceeds the mandated sulfur limit. This can also be an issue if you purchase gasoline from an off-brand station or one with storage tanks that are very old.

If you fill up your car with a tank of gas that has too much sulfur in it, your emissions control system may be overwhelmed and unable to process the excessive amounts of hydrogen sulfide into sulfur dioxide. This results in exhaust fumes that smell like rotten eggs.

Solution: The first and best thing you can do is to use up that tank of gas as soon as possible, preferably on a highway cruise that will heat up your catalytic convertor and burn off the excess sulfur compounds as you drive. Once that you have used up nearly all of the tank of bad gas, refill your car with a full tank of name-brand gasoline from a station in a high-traffic location. If the problem goes away after using some of the new gas, you have solved the problem. If your car still smells like rotten eggs, contact your mechanic, as you may have a more complicated problem.

Manual transmission-related issues

Bad manual transmission oil/gear lube: If you have an older car that is equipped with a manual transmission, it may smell like rotten eggs for a completely different reason. Many manual transmissions, transaxles, and differentials use sulfur-based lubricants because of their ability to stand up to high loads and pressures. If one of these components starts leaking, the lubricants that drip out can produce the distinctive aroma that smells like rotten eggs.

Solution: If you are up to it, you can look underneath your manual-transmission car to see if you can see any fluid dripping from the transmission, transaxle, or differential. Otherwise, simply contact your mechanic and have them examine these components for signs of a leak and the resulting smell. If you have not changed the oil or lube in these components within the recommended service interval, this would be a good time to do it.

Exhaust system issues

Bad exhaust system: If your exhaust system becomes damaged or rusts out, untreated exhaust gases can be escaping from your car, right underneath the passenger compartment! In addition to producing a smell like rotten eggs, there is also the possibility that deadly levels of carbon monoxide can be getting inside your car – and that’s a serious problem. You may also notice an exhaust sound that is louder than normal, coming from underneath your car.

Solution: Open all of your windows and drive your car to your mechanic immediately! Driving around with a faulty exhaust is definitely hazardous to your health. This one can’t wait – you must repair a bad exhaust right away.

Battery issues

Bad battery: The battery in your car is filled with sulfuric acid. Normally, the sulfuric acid stays in its case and doesn’t cause any problems. But if the battery freezes, shorts out, or has a cracked case, the sulfuric acid can leak out, causing a smell like rotten eggs. You may also notice that your car has difficulty starting.

Solution: Carefully inspect your battery, without touching it. If you see that any acid has leaked out or the case is cracked, contact your mechanic and have the problem dealt with. Ask your mechanic about the best way to get your car into the shop – you should not jump start a defective battery and risk an explosion. Safety first!