A sulfur smell can signal an urgent problem. Here's how to figure out where it's coming from and what to do about it.
If Your Car Smells Like Rotten Eggs, Here’s What It Means
Published on Dec. 10, 2024
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Catalytic Converter Failure
The most common cause of a rotten egg smell in a car is a faulty catalytic converter. Catalytic converters are meant to lessen the amount of dangerous gasses your car releases into the atmosphere, and when they fail, they create hydrogen sulfide, which smells like rotten eggs. As a catalytic converter goes bad, it also usually takes in more fuel than normal, as it struggles to process all of the exhaust gasses.
“Sometimes this is a problem that stems from an engine’s fuel system malfunctioning and burning too rich, or, rather, too much fuel,” says Duane “Doc” Watson, a technical trainer at Bosch Mobility Aftermarket. “It could also be an engine mechanical issue, leaving your catalytic converter struggling to convert the unburnt or rich fuel mixture properly.”
Regardless of the reason, it’s a good idea to promptly bring your car into the shop, since defective converters can expose you and the environment to harmful exhaust fumes, says Watson. It can also lower your gas mileage, create poor engine performance and eventually damage your engine.
“It doesn’t always mean replacing the entire unit, though, because it may simply be overheated or clogged,” says Deege Carse, director at Trackershop. “It’s also best to fix the root cause first before replacing the catalytic converter. Otherwise, the issue will recur even with a new catalytic converter.”
Some of the possible root causes include: needing a tuneup, having a vacuum leak, getting bad fuel, having a fuel system that is not at the correct pressure or having an incorrect reading at an airflow sensor, says Chris “Moose” Pyle, an auto expert with JustAnswer.
“Ways to address this are to use some fuel system cleaner, address any stored codes that are in your computer and give the vehicle a tuneup as needed,” says Pyle. “Also, if you’re smelling your catalytic converter, that’s a sign that your exhaust system has a leak, since exhaust odors should only be detected at the tailpipe.”
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Failing Fuel Pressure Sensor or Fuel Filter
Fuel pressure sensors help regulate the amount of fuel that goes into the engine. If one of yours is failing, it could be letting in too much fuel, which is stressing out your catalytic converter. Some signs this might be the case include a car that struggles to start, has declining gas mileage or sluggish acceleration.
If your fuel filter is clogged, it can create similar stress on the catalytic converter and diminish its lifespan. Changing your fuel filter is usually an affordable fix, though, and you should be changing it every 30,000 miles or so anyway.
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Old Transmission Fluid
Old or overheated transmission fluid can also smell like rotten eggs, especially if it’s leaking and burning on the exhaust. To diagnose if that’s the case, look for shifting issues, slipping gears or visible leaks. Also, check the fluid for a burnt smell or dark color and fix it with a flush or leak repair to avoid costly transmission damage.
To keep your transmission fluid in good shape, have it serviced every 30,000 to 60,000 miles, depending on your vehicle’s manufacturer’s recommendations.
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Buildup Inside Climate Control System
Robert says that fungi and bacteria can build up in the vents and ducts of your car’s climate control system, leading to a displeasing odor and, ultimately, the demise of your evaporative core.
“The presence of humidity allows it to thrive and grow pretty rapidly,” he says. “On top of creating a rotten egg smell inside your vehicle, mold can also cause drowsiness, aggravate allergies and asthma and respiratory issues.”
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A Dead Animal
Rodents often use vehicles for shelter and are especially drawn toward the warmth of engines. A sulfur smell could be from them nesting or decaying, says Blair Calder, CEO of Automatic Trap Company. “The smell will not only turn your stomach, but the mice can also do thousands of dollars of damage by chewing on wires and ruining car components,” he says.
To see if mice (or any other dead animal) are the culprits, look for droppings and nesting material, particularly around the engine compartment and air filter. If the smell is coming out of the heater vents, you might need to take it to a mechanic to have them do a thorough check.
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Old or Overheated Battery
If the smell is coming from your battery, it probably means it is damaged and needs to be replaced.
“Overcharged, faulty or leaking batteries can release sulfuric acid fumes, which will stink up your car with a distinct rotten egg odor,” says Danny Stanford-Ravet, founder of Racing Owl Motorsport. “Left unchecked, that smell could soon turn into irreversible damage to your alternator, too.”
Check to see if your battery is visibly swollen, corroded or pushing over 14.5 volts on a multimeter while charging. If it is, replace it immediately, as hydrogen sulfide is highly toxic.
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Bad Gas
Once in a while, the problem can come from the pump, which can affect older cars more severely. If you suspect you got poor-quality gas with a high sulfur content, switch to a higher-grade fuel or a different gas station.
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Old Food
A car smelling like sulfur could simply be coming from rotting food. “It could even be actual rotten eggs,” says Carse. “Maybe a couple slipped to the bottom of your car’s seats during one of your grocery runs.” If so, the good news is it’s a cheap fix, but you will have to do a bit of cleaning.
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Stinky People
Sometimes, it’s not the car that smells like rotten eggs but the driver, says Stanford-Ravet. “After a long day at work, we quickly forget how much sweat and filth we carry,” he says. “A shower is a lot cheaper than a trip to the garage!”
About the Experts
- Duane “Doc” Watson is a technical trainer at Bosch Mobility Aftermarket. He has more than 45 years of experience in automotive repair, has trained thousands of technicians and earned many industry awards, including Chevrolet Technician of the Year and Buick Service Master of the Year.
- Ralph Robert is an expert writer for the Way.com blog and auto app, and an auto enthusiast.
- Danny Stanford-Ravet is founder of Racing Owl Motorsport, with a background in engineering and a passion for all things motorsports.
- Chris “Moose” Pyle is a master-certified technician with 20-plus years of automotive experience. He has also worked as an expert for JustAnswer since 2006.
- Blair Calder is CEO of Automatic Trap Company, based in Sonoma, California, and distributor for the humane Goodnature rat, mouse and squirrel traps.
- Deege Carse is the director at Trackershop and has over 10 years of experience in the industry.
