Emission test failure

I have a 1993 Cadillac Sedan Deville with 179,000+ miles on it. I took it through inspection here in Washington, D.C. And it failed emissions testing. Recently, I had a new exhaust system put on it and a tune up. The HC reading was 440, CO was 4.88 and the CO2 was 9.15. How can I prepare for inspection again? Does the gas quality have anything to do with this problem? I can't think of anything else it could be.