Community / What's That Noise?

A sulfur/rotten-egg smell from the tailpipe, especially at idle or when you lift off the gas, combined with a check-engine light, most often points to the catalytic converter — usually stored as code P0420 ("catalyst efficiency below threshold") if a shop scans it. The catch is that a lazy downstream oxygen sensor or an exhaust leak ahead of the converter can trigger the identical symptom, and those are much cheaper fixes, so it's worth insisting on a proper scan (checking the O2 sensor waveforms) before anyone sells you a converter. It's usually not an emergency — the car is still safe to drive short-term — but it will fail an emissions-related inspection, and a rattling noise from under the car on top of the smell means the converter's internal honeycomb has physically broken apart, which is worth getting looked at sooner rather than later.

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