hey guys i have tried everything i have a 1998 grand prix gtp i have replaced my cat my o2s work fine i checked with a ob2 code reader my egr is opening but only on the highway any suggestions would be awesome all the other emissions passed with flying colours

i think you have answered your own question, the egr is there to reduce NOX, yours isn't working properly.

Roger.

roger roger!!

Egr valve or crack in the erg tube. You didn't do an egr blockoff did you? <grin>