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A rotten egg smell in cars comes from a gas known as sulfur dioxide. The catalytic converter, fuel pressure regulator, fuel filter, etc., can exaggerate this gas if there's a processing fault, creating an odor.

The main causes for a rotten egg smell on a car are:

• Engine oil leak: An engine oil leak can cause a rotten egg smell in a car when the leaking oil comes into contact with the hot surfaces of the engine or exhaust system. As the oil burns off due to the heat, it can produce a sulfurous odor similar to rotten eggs. 
• Fuel regulator leak: A fuel regulator leak can cause a rotten egg smell in a car due to the presence of sulfur compounds in gasoline. The fuel regulator is responsible for maintaining the proper pressure of the fuel system. If the regulator is leaking, it can allow small amounts of fuel to escape, which can then vaporize and cause the smell.
• Differential/transmission fluid leak: A differential or transmission fluid leak can cause a rotten egg smell in a car due to the breakdown of the fluid. Both differential and transmission fluids contain sulfur-based additives that help reduce friction and wear. If there is a leak in either system and the fluid comes into contact with hot components, it'll cause a rotten egg smell.
• Faulty battery: A faulty battery can cause a rotten egg smell in a car due to the release of hydrogen sulfide gas. When a battery is overcharged or damaged, it can produce hydrogen gas as a byproduct of the chemical reactions inside the battery.
• Bad air-fuel mixture: A bad air-fuel mixture can cause a rotten egg smell in a car due to incomplete combustion. When the air-fuel mixture in the engine is not properly balanced, it can lead to inefficient combustion, resulting in the production of hydrogen sulfide gas. This gas has a characteristic rotten egg smell and can be emitted from the exhaust system when the engine is running.

Toxic combustion chemicals can harm humans and damage the car's catalytic converter, which controls emissions. So, even if the check engine light didn't pop, hurry up and take your car for an inspection.

In short, the car's emission system makes sulfur dioxide odorless. Roughly speaking, the catalytic converter transforms toxic gases and pollutants from the engine combustion process into safer-to-breathe gases.

However, when the catalytic converter has an issue, these gases get a free pass to produce bad smells and even cause allergies.

The symptoms might change if the vehicle's tank contains different brands of gasoline. Moreover, even old transmission fluid and the tailpipe's smoke color are clues of a problem.