Electrical smells (think burning toast) could be due to a short circuit in an electrical component, or insulation getting too hot. Take these odors very seriously, because they commonly result in fires.
Musty smells can be a result of mold and mildew in the air conditioning system. As moisture collects on the AC’s evaporator, mildew and mold can form.
Sweet smells can be an indication that you have an antifreeze leak somewhere in your cooling system or heater core. This is important to fix, because antifreeze leaks can lead to overheating issues in your vehicle's engine, as well as being harmful to the human body.
Burning smells usually mean that oil is leaking into the engine or exhaust system, but can also come from an overheated brake pad or rotor. On a manual transmission, the clutch could be worn or overheating, and there is also the possibility of leaves or other foreign material being left in the engine compartment by a human during routine maintenance or by a small animal.
Burning or Burnt Rubber smells normally indicate that a drive belt is slipping or being mangled by a pulley or hose that has broken loose and is hitting a moving part. This could also be a result of a clutch plate overheating.
Sulphuric smells (think rotten eggs), normally indicate that your catalytic converter has gone bad. Usually, an engine or emissions system will have a problem that causes the catalytic converter to overheat, which produces the sulphuric smell.
Gas smells, it can be normal to smell a hint of gas when you first start your vehicle in cold weather, but if you keep smelling it, your gas cap could be loosening, or the emissions control system could be leaking or clogged. Even worse, gas could be leaking from your fuel line or tank! Always investigate these smells when your car is parked before starting it, if possible, to avoid the possibility of igniting the gas.