chevy man chris

I'm new here and this is my first post so I dont fully know how to do this but here goes!
I have a 2003 Chevy 2500 Duramax 6.6l, its chipped but its shut off at the moment, and has about 90k miles on it. Engine light came on, I scanned it it read a P0404 error. I forget which that means but I changed the EGR Valve and I also changed the EGR control circuit. Reason why I changed that is because my mpg's dropped from 18 average to 9-10 average. The engine light went off, its been about a thousand miles so I know the computer reset, engine light hasn't came back on, but i'm still getting 9-10 mpg's. My question to anyone is why am I getting such bad mpg's after i changed both the EGR vavle and EGR control circuit? Could it be that since the EGR system faulted then it clogged up the cat's because they weren't able to clean out? Any suggestions would be appreciated thanks