I, too, have an '04 Sentra and the check engine light would come on, stay on for a week or so and then go out--only to recur a week or so later. My car had a code 1138. Took it to the Nissan dealer's service dept. They told me that the Swirl Control Valve was bad and that replacing it required replacing the intake manifold, as it is integrated into the manifold. The good news was that it was covered under either the Emission Controls Warranty or the Powertrain Warranty, which is longer than the 3yrs/36K general warranty. The Swirl Control Valve is an emission control part, but, since it is integrated into the intake manifold, I don't know which warranty they covered it under. If you call Nissan USA headquarters, they will tell you if the warranty is still in effect at 66K miles or not--and I'm sure they could tell you which warranty, if either, applies. Hope you're still covered, as it isn't a cheap repair.