The smell like rotten eggs in a car is typically caused by hydrogen sulfide gas, which is a sulfur compound. The most common reason is a malfunctioning catalytic converter, which normally converts hydrogen sulfide into odorless sulfur dioxide. When the catalytic converter is faulty or clogged, it cannot properly process the exhaust gases, leading to the rotten egg smell. Other possible causes include a damaged or overheating battery releasing sulfuric acid fumes, clogged or dirty cabin air filters with mold or bacteria, old or bad fuel high in sulfur, or even mold, fungus, or dead animals in the air vent system. Here are key causes in brief:
- Faulty catalytic converter not converting hydrogen sulfide correctly, often due to excess unburnt fuel caused by engine or fuel system issues.
- Overheated or damaged battery emitting sulfuric acid fumes.
- Mold or bacteria growth in the air conditioning or ventilation system.
- Clogged cabin air filter.
- Bad or sulfur-rich fuel.
- Decomposing food or dead animals inside the car or engine compartment.
The rotten egg smell often becomes noticeable after the car is warmed up, and it might be strongest around the exhaust or inside the cabin if the exhaust leak is present. Immediate inspection and repair are recommended, especially for the catalytic converter or battery, as these can impact car performance and safety. If the smell occurs specifically when the air conditioning is on, consider the AC system mold or a clogged drain line as possible causes. Overall, the catalytic converter is the most common culprit, but it is important to check the battery, fuel system, air filters, and even look for possible mold or foreign organic matter inside the car.
