What’s That Smell Coming From My Car’s Engine?

Spill something in your car—say, coffee or soup—and you’ll have a pretty good idea of what’s causing any strange scents. (Here’s how to get rid of those car smells.) But if the origin of an odd odor isn’t clear, chances are it’s coming from the engine, says Jill Trotta, vice president, industry advocacy and sales at RepairPal and an ASE-certified technician—and it’s not a good sign.

“Car smells are a clear indicator that’s something wrong,” says Trotta. “So you never want to ignore a strange smell when you’re driving.” Here are some of the most common car odors, she says, and what to do if you pick up their scent.

If Your Car Smells Like Rotten Eggs

What it might be: Catalytic converter

“A sulfur smell could indicate an imbalance in your engine’s air-to-fuel ratio,” says Trotta. If you have fuel-injection problems, the unburned fuel can plug your catalytic convertor, and the exhaust will have nowhere to go. End result? Your car won’t run.

What to do: Have a mechanic look at your car immediately. You can save the catalytic converter if you catch the problem early enough.

If Your Car Smells Like Mold Or Mildew

What it might be: Air-conditioning

Your car’s air-conditioning works by pulling moisture out of the air; the extracted water goes into a box behind the dashboard, which has a drain. Leaves or papers can work their way into that box and block the drain. The standing water will eventually get moldy, which is a costly problem to repair. (Plus, the water will often find another way out of the box—usually onto the carpets and floor mats.) Mold-spawning moisture can also build up in your duct system unless you regularly run your A/C.

What to do: If you smell mold, have it checked out. To help prevent moisture buildup, run your A/C every month or two for a few minutes—even if you don’t need to cool off. This also helps lubricate oil seals and bearings in the compressor, which will make it last longer.

If Your Car Smells Like Maple Syrup Or Fruity Candy

What it might be: Coolant

A sweet smell coming from your engine is probably coolant escaping from the cooling system, which means you probably have a leak. That could wreak havoc on your car, including causing your vehicle to overheat.

What to do: If coolant is leaking, you could damage the engine by driving. Have your vehicle towed to the nearest repair facility.

If Your Car Smells Like Acrid Smoke Or Burning Oil

What it might be: Oil leak

An oil leak is dangerous for two reasons. First, if it hits the exhaust, a fire could result. Also, a low oil level could damage the engine. If you notice the smell not long after an oil change, the leak might be caused by a loose drain plug or filter that wasn’t properly attached—or an oil cap that wasn’t screwed on tightly enough. An oil leak from a bad gasket or seal can cause problems, such as oil dripping on the timing belt or the crankshaft seal. Either way, it’s not something to ignore. “It could totally take your engine out,” says Trotta.

What to do: Take your car back to the mechanic and have the oil level checked.

If Your Car Smells Like Burnt Carpet

What it might be: Overheated rotors or brake pads

If you’re going down a steep hill and keeping your foot on the brake, even lightly, this smell may indicate that the brake pads or rotors are overheating. This can cause premature brake wear or, in extreme cases, brake failure. (Here are 5 signs you need new brake pads.) The smell could also be caused by dragging or sticking brake calipers, or brake pads that are too thin (in which case you may feel like the brakes are “spongy”). If the smell persists and you haven’t been driving in stop-and-go traffic or on long downgrades, have your brakes inspected ASAP. Also, be aware that if you’ve just replaced your brake pads, it’s normal for them to release a scent for the first couple hundred miles.

What to do: Pump the brakes on and off when going down hills.

If Your Car Smells Like Exhaust Or Fumes

What it might be: Exhaust leak

Leaky exhaust that gets into the car’s interior can quickly raise the amount of carbon monoxide to dangerous levels.

What to do: Roll the windows down immediately and pull over to the side of the road when you can do so safely. Turn the engine off. Have the car towed to your repair shop.

If Your Car Smells Like Burnt Rubber

What it might be: Slipping belts or overheating rubber

This may be the result of an engine belt that’s slipped. It could also be a hose from the cooling or power steering system that’s rubbing on a belt and has started to melt.

What to do: Wait until your car cools down, then open the hood and inspect your belts and hoses. If the smell persists, take the car in to be checked out.

If Your Car Smells Like Burning Plastic

What it might be: A short circuit in the wiring

When the plastic insulation has worn down (or been chewed off by an animal that’s gotten into the engine), the exposed wires can rub together and produce a short, which could cause a fire. Other types of shorts can melt or burn the plastic away directly. However, it may be as simple as a plastic bag that’s landed on the exhaust and melted from the heat.

What to do: Pull over safely. If the problem is a bag or some other plastic on the exhaust, don’t grab it, as it’s likely very hot. If you can’t locate the smell, or it persists, take your vehicle in for diagnosis and repair.

If Your Car Smells Like Gas

What it might be: Gas leak

This is one of the most dangerous smells and could mean that something is leaking in the engine or near the exhaust, like a fuel line. A fire could start if the raw fuel comes in contact with the hot exhaust system or rotating engine components. If you have a car from the 1970s or earlier, a lingering fuel odor after you turn off the engine might be normal; if that odor is strong, however, don’t wait to get it checked out.

What to do: Pull over safely and turn off the engine immediately. Get your car towed to the auto shop.

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Read more: Winter’s coming—help prepare with our extreme weather driving guide.

By Ellise Pierce