Exhaust Smells Like Sulfur: Rich Running, Catalyst Stress, or a Converter That Is Finally Failing?

A sulfur or rotten-egg exhaust smell usually means the catalyst is processing a rich or contaminated exhaust stream, not that the smell by itself proves the converter is the root cause. The smart job is to decide whether the converter is reacting to upstream fuel-control trouble, overheating after misfire, or has actually lost efficiency and started telling on itself.