Bad odor from exhaust

I have a 2004 Denali which is emitting a sulfur/swampy smell from the exhaust. I have had it in the dealership several times. They have replace the catalytic converter and the to oxygen sensors upstream from the converter. The problem still exists. Does anyone have any suggestions?

The most likely cause of a stinky exhaust is a rich mixture. The O2 sensors are usually the problem, but not always. The smell is caused by the sulfur in the fuel when the rich mixture is catalized in the cat. You need to take it back and get a real tech, not a R&R (remove and replace) tech. They should be able to hook it up to a OBDII decoder and see the sensor readings real time. The O2 sensors downstream of the cat are to check the operation of the cat and should read lean, if not you have a rich mixture.
It could be a bad injector, computer, wiring from the sensor to the computer ect. They need to diagnose the problem before they start changing parts.
Check out:
http://www.scantool.net/
they have an excellent decoder for under $100 and will allow you to read out all of the codes, as well as all of the the sensor outputs realtime.
A little information in the hands of a customer can be a dangerous thing to a service manager.
Let us know what you find. good luck.
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