Had the local auto Zone read the codes. Turned out it was an EGR valve. Ten minutes of work and all is well. Darned EGR valve was pricey though, at Auto Zone, $212.00 incl. tax. called the Chevy dealership and they wanted $255 + they would have it Monday afternoon? ... guess I just never paid much attention to prices for parts and things for the newer vehicles since I never worked on them. Give me an old 60's or early 70's vintage engine anyday...