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Pee-yew! Why does my exhaust smell like rotten eggs?
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For the same reason rotten eggs smell like rotten eggs:  Sulfur. (Also spelled "sulphur")

The catalytic converter (and water in the exhaust) react with the fuel's sulfur, causing the bad smell. The problem primarily affects cars with very new cats, and which live in areas with high sulfur content in their fuels. Over time, the catalytic converter will get coated from normal use, its efficiency will be reduced and the smell will go away.

Sulfur will normally NOT damage the engine, and will NOT affect your ability to pass emissions tests. However, there are rare circumstances where the problem can be severe enough to cause damage to the catalytic converter. Since the car or cat will be under warranty, the best thing to do then is to take it in to the dealer and have the pros tell you everythng's OK.

This page gives the details of the issue.
An excerpt from that page:

"Most gasoline sold in the United States has fairly high sulfur
levels. The national average is 350 parts per million (ppm); as much
as one-fourth of gasoline has a sulfur level of 500 ppm or higher,
according to petroleum industry figures. On the other hand,
California, the nation's smoggiest state, now has a statewide sulfur
standard of 30-40 ppm. California's standard allows refiners to
produce gasoline in batches that meet a sulfur content average of 30
ppm, so that in some cases, sulfur levels in gasoline can be as high
as the state's sulfur cap of 80 ppm. Most northeastern states, also
suffering from high ozone levels, sell reformulated gasoline with
sulfur levels of 150 ppm"

Here's a Toyota TSB saying much the same thing.

It's very expensive to remove naturally-occurring sulfur from fuels. Since all the pressure for removal comes from government regulation, refiners are reluctant to voluntarily be the first to undergo the necessary large expenditure until everyone else is forced to face the same big bills. Motor fuels are fairly low-profit commodities. Nobody wants to be the only one stuck with unique heavy expenses in a low-margin business. Eventually, tighter sulfur regulations will cover all of North America, and the rotten egg thing will be gone forever (and your fuel will be just a bit more expensive!).

So what to do until the smell goes away? Different brands, even different grades within the same brand, may contain differing amounts of sulfur. All you can do is switch brands and grades until you find one that does not stink. But by the time you do find one, the smell may be gone anyway.