I recently bought a 96 Z28 and the check engine light came on and it threw off an error code about the auxilary emission control manufacturer control and i have no idea what thats about. I replace the egr valve and its still giving the same code. when i had the egr off i experianced a gain in HP and performance which says to me a vaccume leak some where but i can't find one. Is there a secondary emission system somewhere with a possible sensor out? Any input would be a huge help.

 

One of your O2 sensors that sit after the cats could have gone bad, I believe it also throws a code when the A.I.R. system isn't working properly.