Has the Check Engine light been on?
Rotten egg smell means the vehicle is running rich.....
Is this vehicle misfiring?
As for reading codes, you could be in a transition year....do you have a 16 or 12 pin ALDL? If 16, no flash codes available....if 12 pin, you ONLY have flash code capability if there are pins in the "A" and "B" connectors....chances are on this year you don't.....because it is a transisition year, many scanners can't read the codes in this vehicle...it would take a GM scan tool, or an expensive advanced one to read them.....
Yes a contaminated O2 sensor could cause the vehicle to run rough......however, just changing the sensor may not solve your problem, because whatever caused that sensor to go bad(for example, running rich), is still there and will contaminate the sensor again......
Last tuneup on vehicle?