good friend of mine has every state emmissions certification availible in PA. i asked him about the rotten egg smell as my gf's car was emmitting it. i've never seen his eyes roll back so hard. the rotten egg smell does NOT mean your cat is bad. it's usually a sign that you picked up some bad gas OR your car doesn't want to run on the octane you're giving it. we started putting 89 octane in her car instead of 87 and the rotten egg smell disappeared.

only way to prove your cat is bad is to get an emmissions test done.