It sometimes happens that an odor appears in the passenger compartment of the vehicle, what smell for what dysfunction?

Bad odors can lead to costly repairs or health risks, so be sure not to ignore them. Here are some common odors and their possible causes.

Musty

If turning on the air conditioner generates a musty smell, mold and/or mildew have 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) and it may be harboring mold. Running only the fan at high speed (with the air conditioning off) can dry the evaporator.

Burning

Oil could be oozing onto a hot part of the engine or exhaust system. It also could come from overheated brake pads and/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.

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 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.

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.

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 gas when a cold engine is first started because of incomplete combustion. If you smell gas 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.