How to Identify the Cause of a Rotten Egg Smell in Your Car

• The rotten egg smell is probably a sulfide created by your engine. If you smell it, it means your vehicle’s emissions system has an issue.
• Most of the time, a bad catalytic converter is the core reason you smell rotten eggs, so see a mechanic to get your catalytic converter replaced.
• In rare cases, a leaking battery, dead animal in the vehicle, or transmission issues could cause rotten egg odors.

Most Likely Causes of a Rotten Egg Smell

Your catalytic converter is clogged or broken. The catalytic converter is responsible for absorbing harmful emissions created by your engine, so if it’s damaged or at the end of its life, those emissions can build up and flow back into the cabin. In almost all cases, those emissions will smell like rotten eggs since a car’s main emissions are hydrogen sulfide—which smells kind of like eggs. The Fix: You need a new catalytic converter. See your mechanic and ask them to take a look. If you don’t replace your catalytic converter, you’ll eventually do serious and potentially irreparable damage to your engine all while spreading toxic emissions into the environment. Just as a heads up, a new catalytic converter is one of the more expensive repairs out there. You may end up sending up to $2,500 on a new converter.

Your fuel pressure sensor is failing. The fuel pressure sensor regulates how much fuel flows into your engine from the gas tank. If the sensor isn’t working, you’ll end up with too much oil flowing to the engine, and those fumes will burn off to the catalytic converter. The converter won’t be able to handle the massive flow of fumes, and they’ll flow back into your vehicle’s cabin. You’ll also notice black smoke coming out of the exhaust and bad fuel mileage if the sensor is bad. In fringe cases, a blocked fuel filter or damaged fuel pressure regulator can cause that egg odor. Those issues are more likely to cause your car to smell like gas, though. The Fix: The fuel pressure sensor is a very tiny component that requires special tools to calibrate. See a mechanic and ask them to test and replace the sensor.

Your car battery is leaking because it’s going bad. Your car battery contains a variety of lead and acid compounds that work together to exchange electrons and generate charge. However, if your battery is damaged, that lead-acid mixture can leak out, and those chemicals often resemble rotten eggs. Do not jump a bad car battery. Jumping a leaking car battery can cause it to explode. The Fix: Replace the battery. Purchase a replacement battery, disconnect the old terminals, and unscrew the battery from the mount. Put your new battery in place, connect it to the mount with the screws, and assemble the terminals.

Your transmission fluid needs to be changed. Your transmission fluid is responsible for lubricating the gears in your transmission. If that fluid is leaking or going bad, the friction in the transmission will burn off the residual fluid which can cause a rotten egg odor. Bad transmission fluid will also cause weird grinding noises, so keep your ears open. The Fix: If you’re a gearhead, drain and change the transmission fluid yourself. If you spot any leaks or the pressure drops in your transmission fluid after changing it, you have a leak in your transmission line. See a mechanic to get the line patched and repressurized. You typically need to change your transmission fluid every 30,000–60,000 miles (48,000–97,000 km).

More Uncommon Sources

Your manual transmission is leaky or damaged. If you’ve got an old manual transmission, those gears get a lot of wear and tear. If they aren’t properly lubricated, the oil in your transmission could burn off and cause an egg-like odor. The Fix: You likely need to get your transmission rebuilt or replaced by a professional. You can try troubleshooting it yourself, but manual transmissions can be tricky for a non-pro.

Your exhaust system is leaking. The exhaust system is responsible for funneling all the emissions your engine creates to the catalytic converter. So, if your exhaust system has a leak, you may smell rotten eggs around the vehicle when it’s running. If your exhaust system is leaking, you may also notice water dripping from the exhaust pipe as moisture gets into the system. The Fix: See a mechanic just to make sure your catalytic converter isn’t the damaged part. They’ll patch your exhaust system for you.

An animal died in your car. Squirrels, rats, and possums will often hide themselves in the undercarriage, engine bay, or exhaust pipes—especially if it’s cold or rainy out. If an animal happens to die in your vehicle though, it can smell like rotten eggs as the body decomposes. The Fix: Pop the hood and take a close look around the engine bay for any animals. Then, look at your undercarriage and inside of the exhaust pipes. If you do find an animal, put on gloves and remove the corpse. Air out the inside of your vehicle after removing the animal to get rid of any residual odors.

Is it dangerous to drive if I smell rotten eggs?

It’s probably not dangerous, but it’s not good to leave this problem off. Your vehicle is extremely unlikely to catch fire or leave you stranded if your vehicle smells like eggs, so it’s not explicitly dangerous. However, it’s a sign something is probably off with the emissions of your vehicle, so either your engine or transmission are running suboptimally and you’re putting unnecessary wear and tear on your vehicle, or you’re unleashing toxic emissions into the air. Both aren’t great, so don’t leave this repair off for too long. If you want to play it safe, call a tow truck to get your vehicle towed to the mechanic to avoid putting stress on your engine or transmission.

Why do I still smell eggs after fixing the problem?

The odor may be trapped in your cabin air filter, so swap it out. The cabin filter, which is normally in the glove box, is the filter that soaks up emissions that might accidentally wander into your interior from the engine. If the rotten egg odors are still present after you’ve fixed the underlying problem, there are probably some smells trapped in the filter. Purchase a replacement and look up the process to change the filter based on your make and model. The way you change the cabin air filter differs from vehicle to vehicle. Typically, you unscrew a panel inside of the glovebox and slide the filter out.