Here are some common bad car smells and their possible causes:
Musty: If turning on the air conditioner generates a musty smell from the vents, mold or mildew has probably formed in the air-conditioning system. Moisture naturally collects on the cold air-conditioning evaporator (a small radiator that carries refrigerant into the car's dashboard through the vents), and it may be harboring mold. Running only the fan at high speed (with the air conditioning off) can dry the evaporator to clear the mildew and rid the smell.
However, that doesn't guarantee the problem won't reoccur — especially if the odor is being caused by a clog in the drain tube that allows water to drip out under the car. A musty mildew smell also can be caused by carpets that get wet when water leaks into the interior.
Sweet: Antifreeze has a sweet, syrupy odor, and smelling it inside a car usually means there's a leak somewhere in the cooling system. The source may not be easy to see. For example, the leak could be from a corroded heat exchanger (aka a heater core), which is usually behind the dashboard. The leak could be in the form of steam that enters the cabin, producing the smell and potentially fogging the windows. Have this problem addressed, because breathing antifreeze isn't good for you.
Burning: Oil could be leaking onto a hot part of the engine or somewhere in the exhaust. It also could come from overheated brake pads or rotors — due either to aggressive braking, pads that don't retract when you release the brake pedal or the emergency brake being left on while driving. On a vehicle with a manual transmission, the clutch plate could be worn or overheating from riding the clutch pedal. Leaves or other material in the engine compartment — sometimes imported by nesting rodents — also can burn on hot surfaces.
Rotten eggs: If you can smell rotten eggs or sulfur, your catalytic converter may have gone bad. The root cause could be an engine or emissions system problem that made the converter overheat, thus causing the rotten egg smell.
Rubber: The smell of burning rubber could be an accessory drive belt that's slipping or getting chewed up by a broken pulley or hose rubbing against a moving part. An overheated clutch plate also can smell like burning rubber.
Electrical: Smell burnt toast? That could be a short circuit in an electrical component or overheated insulation. Take electrical odors seriously, because short circuits and overheated components are common sources of fire.
Gas: It's normal to smell a little gasoline when a cold engine is first started because of incomplete combustion. If you smell gas fumes after the engine is warm, though, the gas cap could be loose or the evaporative emissions control system — which is supposed to contain fuel vapors and recycle them through the engine — could be leaking or clogged. Even worse, gas could be leaking from the tank or another part of the fuel system, such as the fuel line. Always investigate gas smells you discover when your car is parked before starting the car and potentially igniting the fuel.
Rotting fruit: It's probably what it smells like. Look under the seats for a decomposed apple or banana on the carpet. Be sure to clean the affected areas with a spray bottle or cleaner to clear out any resulting bacterium.
