What Causes the Smell of Rotten Eggs in a Car?
3 Answers
The smell of rotten eggs inside a car is mainly caused by hydrogen sulfide in the exhaust. Additional details are as follows: 1. Source of the odor: The rotten egg smell in the exhaust often occurs when the car is cold or just started. At this time, the three-way catalytic converter is at a low temperature and has not reached its normal operating temperature, thus failing to perform its catalytic function, resulting in higher emissions of hydrogen sulfide. The higher the hydrogen sulfide content, the stronger the rotten egg smell. If the rotten egg smell persists for a long time and is very strong, it may indicate a problem with the three-way catalytic converter, engine oil, or the engine itself. In such cases, it is advisable to visit a professional automotive service shop for further inspection. 2. Solutions: Open the windows for ventilation to remove the odor; place bamboo charcoal bags; install a car air purifier; use tea leaves to eliminate odors.
Last time I had this smell in my car, I immediately looked it up—it's actually the most common sign of a battery problem! Lead-acid batteries release hydrogen sulfide gas when overcharged or aging, which smells exactly like rotten eggs. Especially in the summer heat, the chemical reactions inside the battery intensify, making that weird odor more likely to drift into the cabin. If you also smell burning plastic at this point, be even more alert—the battery might be swelling and close to bursting. It's recommended to immediately turn off the engine, pop the hood for ventilation, and prevent the battery from overheating further. At the same time, check the battery terminals for any white crystalline deposits and replace the battery promptly to eliminate risks. Regular maintenance and keeping an eye on the battery's condition are crucial—don't wait until you're stranded on the road to regret it.
Over the years repairing cars, I've encountered this situation quite often, and 80% of the time it's caused by the exhaust system. The hydrogen sulfide produced during engine combustion is converted into harmless gases by the catalytic converter. But once the converter fails or the sulfur content in the fuel exceeds standards, that pungent hydrogen sulfide gets mixed into the exhaust and drifts back into the cabin. You'll notice the smell is especially strong during acceleration because the engine emits the most under high load. Another possibility is a leak in the exhaust pipe gasket—just like when my home's water pipe burst—allowing exhaust gases to seep directly into the cabin through the gaps. I recommend getting an exhaust gas test and having a mechanic check the exhaust pipe seals with a smoke tester. Usually, replacing the catalytic converter or resealing the pipes will solve the issue.
