Rotten egg smell from the exhaust or engine

Symptoms

• Distinct rotten egg or sulfur smell from the exhaust, especially noticeable at idle or in traffic
• Possible reduced engine performance or poor fuel economy alongside the smell
• Check engine light sometimes illuminated
• Smell may be worse when the engine is under load or after long idling

Likely causes

• CommonFailing or clogged catalytic converterThe catalytic converter is supposed to process sulfur compounds in exhaust gas; when it's failing or overwhelmed, it can let this rotten egg smell through.
• CommonEngine running rich, overworking the catalytic converterAn underlying rich-fuel condition (see the black smoke guide) increases the sulfur load, more than a healthy converter can handle, causing the smell even if the converter itself isn't yet fully failed.
• PossibleLow-quality or high-sulfur fuelOccasionally a specific batch of fuel can cause a temporary rotten egg smell that clears after that tank is used up.

What to do

1. Have the check engine light's codes read with a scan tool if it's illuminated, since codes related to catalyst efficiency or fuel trim point directly at the cause.
2. Note if the smell coincides with any of the black-smoke or rough-running symptoms described in related guides, since a rich-running condition often accompanies converter issues.
3. Try a full tank of fresh fuel from a different station if you suspect a bad batch, though this is a less common cause.
4. Have the catalytic converter inspected and tested for efficiency by a mechanic, since replacement is the fix if it's genuinely failed.
5. Address any underlying rich-running cause (sensors, injectors) at the same time, since a new converter can fail again quickly if the root cause isn't fixed.

Tools and parts

• OBD-II scan tool

When to call a pro

See a mechanic with scan tool access for a rotten egg smell, since diagnosing whether it's the converter itself or an underlying rich-running cause needs proper testing.

Safety