first you need to know that there is a cat. and a pre-cat. the pre-cat is in the header. the cat is in the downpipe.
deleting the pre-cat will not eliminate the smog control on your car (still has cat). but if you install an aftermarket header, then you will need an o2 spacer if you don't want the code to come back or you can make a second o2 sensor hole on your downpipe after the cat (or get stock downpipe from an 04-06 sentra)
you have two o2 sensors. both on header, one before and one after pre-cat. the first controls the A/F ratio and the second is there to validate the first and to detect the cat (or in the 02-03 sentras, the pre-cat) which it does by getting a different A/F ratio reading than the first sensor. so with after market header, you use the o2 spacer to pull the sensor out of the direct exhaust flow so it tricks ECU into thinking cat is working.
unfortunately, if your pre-cat is toast, you most likely have more problems than the just code it's throwing. QR25DE Precat and Butterfly Screw FAQ - B15sentra.net - Nissan Sentra Forum