Rotten Smell from Exhaust

I have a 2004 Denali which is emitting a bad rotten egg or sulfur odor from the exhaust. The local GMC dealer has replaced the catalytic converter and this did not fix the problem.

Can anyone provide information regarding a fix for this problem?

Change the Oxygen (O2) Sensors, the ones in front of the CAT (upstream). There should be 4 O2 Sensors in your exhaust system, two on each exhaust pipe before the cat and two after.
The Oxygen Sensor sniffs at the exhaust to determine how much oxygen is left after combustion. It generates a voltage from around .2V for a rich mixture to .9V for a lean mixture. This voltage is used by the engine computer along with other inputs to control the injector on time, timing, etc.
The problem is that these sensors sometimes drift and give the computer phony readings and as they say garbage in, garbage out. If the sensor has drifted to a lean reading the computer will command extra fuel to correct the "lean" condition which will be left in the exhaust to be burned in the cat and produce Hydrogen Sulfide and other wonderful swampy smells.
The two rear O2 sensors tell the computer if the cats are working and don't have a great deal to do with the moment to moment operation of the engine.
Hopefully you’re still under warrantee since these puppies are about $100 each. Take it back to the dealer and tell him that you think there may be a problem in the upstream oxygen sensor, speak slowly and loudly, he can hook his computer up to the truck and test the sensors or try replacing them as a test.
Let us know what happens
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