Exhaust, fuel & emissions

Rotten egg smell from the exhaust

A rotten-egg or sulfur smell from the exhaust almost always means the catalytic converter is struggling to process hydrogen sulfide, usually because the engine is running too rich or the converter itself is failing.

Difficulty
Intermediate
Time
30–90 minutes to diagnose
DIY cost
$0–$150
Pro cost
$150–$1200

Symptoms

  • Sulfur or rotten-egg smell, strongest at idle or after driving
  • Smell noticeable near the tailpipe or when windows are down behind the car
  • Possible loss of power or rough running
  • Check engine light may be on

Likely causes

  • CommonFailing or overloaded catalytic converterA converter that's aging or clogged can't fully convert sulfur compounds.
  • CommonEngine running rich (too much fuel)Excess unburned fuel overwhelms the converter and produces the sulfur smell.
  • PossibleFailing oxygen sensorA bad sensor can cause the computer to keep the mixture too rich.
  • Less commonLow-quality or high-sulfur fuelSome fuel batches have higher sulfur content that briefly worsens the smell.

What to do

  1. Get the check engine light scanned for rich-running or converter-related codes.
  2. Check for other signs of a rich mixture: black exhaust smoke, poor fuel economy, or a strong gasoline smell.
  3. Have the oxygen sensors and fuel trims checked by a shop.
  4. Have the catalytic converter inspected for clogging or damage if the rich condition is ruled out.
  5. Try a full tank of fresh, quality fuel if the smell appeared right after fueling up.
  6. Address the root rich-running cause before replacing the converter so the new one doesn't get damaged the same way.

Tools and parts

  • OBD-II code reader
Oxygen sensorSearch: “oxygen sensor for [year make model]”
Catalytic converterSearch: “catalytic converter for [year make model]”

When to call a pro

See a shop if the smell persists past a tank of fuel or comes with a check engine light, since a rich condition left alone can ruin a new catalytic converter too.

Safety

Generally low risk

Generally low risk for a careful person. Stop if anything looks different from what this guide describes.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.