boggy take off

I bought a 2000 Tahoe w/ 5.3 ran it for about a month, did a tune up, plugs, wires, oil ect. I also put on a cold air kit, and flowmaster exhaust. I'm now pulling a p0420 code. I am new to the mechanic side and learning as I go. I know what the code is, so I did a diagnostic pull and am...