I need some help, just bought my 1998 c1500 ext cab a few months ago and last week I got the P0420 code when I had the MIL checked. A bit of background on the truck-1.)New 5.0 engine 4k mi. on it, 172k on truck. 2.)it seems to run fine but noticed a slight hesitation every once in a while. 3.) it has a K&N cold air kit 4.) every thing under hood appears brand new (alt, wires, batt etc.) 5.) I checked for exhaust leaks and noticed that this doesn't seem to be the original cat (sloppy welds) but no leaks. Any help would be appreciated!

 

Replace the cat with a OE part.