CEL and Long tubes and Y pipes.
 

I want to get Long tubes and a ORY pipe. Im confused if it will throw a CEL and if it does do i NEED to have it tuned out or does the rear o2 defouler work just fine and not hurt and performance?

5.3L 2000 Silverado btw

Yes it will throw a code... you will have to have them turned off via tuning software. But you will want to tune for the headers anyway so turning the rear 02s off will be no problem.