I have a 1998 Chevrolet z71 5.7l v-8 with 194000. My check engine light came on today. I was able to get the code which was P0420. It says that I have one of four problems. One, catalytic converter defective. two, engine misfire or running condition. Three, large vacuum leak. Four, engine oil leakage into exhaust valve guide, piston rings or seals. The truck seems to run and idle fine, starts on first crank and idles fine. I am curious what I can do to eliminate the problems to isolate the issue as I am not very knowledgeable about much more than oil change or brakes. Any help would be appreciated. Thanks