I have failed emissions 3 times. first test I failed HC and CO but passed NOX. Second test I just cleaned out the throtthe body, removed my hypercrap chip/ replaced with stocker and ran 2 bottles of dry gas and the damn HC went up. The last test I replaced O2 sensor, changed oil, distributor cap & rotor, PCV valve, checked spark plugs & wires-they looked good, and I failed all three (NOX went through the roof) HC & CO went down but not enough. I think the new O2 sensor is bad?

I do use a lot of oil but no more than usual and I have passed before. Now my exhaust smells like sh*t. Maybe Cat Converter should be repalced next. I do have to run high octane or I get ping. There is black smoke when I get on the throttle which makes me think it is running rich. My mechanic checked the timing and said it was good. How do I check for a rich condition? Where should I be looking next? EGR? How do I check that?

 

Have that mechanic check your EGR. He can hook a vacuum source to it and see if it opens and closes properly. I assume he was a competant mechanic and checked your timing right. I have had GM dealerships not be able to do it right if that tells you anything. Many people forget to unplug the ESC. Your converter could be the cause but I would have him check for the functionality of your EGR.