I've been having a problem off/on for months now.....I keep getting s**t mileage(19-21 mpg) and a "rotten egg' smell every so often. i figured I'd first try that EGR cleaning from SE-R.net....I then changed-fuel filter, spark plugs, ignition wires, pcv valve and last but not least, O2 sensor. After all that, I still have a "skip' at partial throttle, still enconter that sulfur smell now and again, and even worse, my mileage is still less than 25mpg. Thw wierd thing is, I figured that my catalytic might be shot, but everyplace around here carries either some POS "direct fit replacement"(where it doesn't have the same pipe diameter as stock, even) or it's a weld-on unit, with moi' having to track down a shop that isn't going to charge 245$ for the Catalytic and the welding.(that's the lowest price I've been quoted). The worst part is, a few mechanics I've spoken with on the phone seem to feel that it isn't the 'cat'-they say that Sentra's cats are pretty resilient....I just need to "tune" my engine. WTF!?!? Didn't I just do that with all the other s**t???