A rotten egg smell from your car is unpleasant and a clear signal that something is wrong. Here's what causes it and how serious it is.
The Most Common Cause: A Rich Fuel Mixture
Gasoline naturally contains sulfur compounds. When the engine runs rich (too much fuel, not enough air), unburned sulfur-containing compounds exit through the exhaust. Your catalytic converter normally neutralizes these compounds, but if the converter is overloaded with unburned fuel, it can't keep up — resulting in that distinctive rotten egg (hydrogen sulfide) smell.
What Causes a Rich Mixture?
- Faulty oxygen sensor — Sending incorrect data to the ECM, causing over-fueling
- Failed MAF sensor — Mis-measuring air intake, leading to excess fuel delivery
- Leaking fuel injector — Allowing raw fuel into a cylinder even when not firing
- Worn spark plugs — Incomplete combustion leaves unburned fuel in exhaust
Catalytic Converter Issues
A failing catalytic converter itself can also produce a sulfur smell — particularly one that's been damaged by persistent misfires or rich running conditions. At this stage, both the cause and the converter may need replacement.
What to Do
Start with a diagnostic scan to read fault codes. Codes pointing to oxygen sensors (P0130–P0167) or fuel trim issues (P0171, P0172) give you a starting point. Addressing the root cause early prevents catalytic converter damage.
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