I have a 1999 Dodge Ram 1500 4x4 5.9L. I keep getting code po420. I know what this means, so here is my question. Does the code keep coming back because it is searching for the catalytic convertor? It has one but I busted all of the stuff out of it and put it back on because it was clogged. I have replaced both the pre and post o2 sensors. I keep clearing it with my scanner and after several days I get the code again. Sometimes a week or two may go by before it comes back on. The engine runs fine other than my gas millage has dropped which I have expected with no convertor. Any advice or suggestions would be greatly appreciated. Thank you

 

http://www.obd-codes.com/p0420

your computer is reporting that your gutted cat is not working properly. specifically, the rear O2 emissions readings are not lower than the front.

there's a couple of things you can try.
one is to install a spark plug non-fouling spacer to move the O2 a little farther out of the exhaust pipe. some have success with this.

the other is to remove the rear O2 and strap it up in open air. plug the hole. this works for others.

 

Non fouler worked for me. Cheap too.

 

Or put a new cat on.

 

+1... And fix the plenum. (the reason the cat went bad to begin with)

 

+1...or would that be +2 (and fix the plenum...)

 

Thanks for the suggestions. I will be giving those a try.