If your vehicle is showing burning sulfur or rotten egg smell coming from under the car, here are the most likely causes and the parts you'll need to fix each one.
The catalytic converter is designed to neutralize exhaust gases. If it fails, or if the engine runs too rich and dumps unburned fuel into the exhaust, the converter overheats and fails to process the sulfur in the fuel properly, creating a rotten egg smell.
A failing alternator or voltage regulator can overcharge the battery. This causes the sulfuric acid inside the battery to boil and vent highly toxic hydrogen sulfide gas, which smells exactly like rotten eggs.
Gear oil and manual transmission fluid contain extreme-pressure additives heavily laden with sulfur. If a seal leaks or the fluid was overfilled during a recent service, it can drip onto the hot exhaust pipe and burn off, creating a strong sulfur odor.
Faulty fuel system components can dump excess raw fuel into the combustion chamber and subsequently the exhaust. This overloads the catalytic converter, causing it to overheat and emit a sulfur odor.