"I wish that before I had bought my 2.5S I had known about all of the engine problems. Google the QR25DE Altima engine and look at the forums. There is a pre-catalyst recall - the problem can mean anything up to a new engine. But at least that is recalled, so covered by Nissan.
The WORST issue in an 02 or 03 is the butterfly screws in the intake. They work loose and make their way into your engine and cause damage. They recalled this in 04 and 05, but it is the exact part as the 02 and 03, but no recall. So my car is at Nissan right now because a butterfly valve screw made a journey through my engine, but I have to pay for the damage.
I love my car, but BUY THE V6!!!!!!!
Incidentally, Nissan uses this engine in the Sentra and some Frontiers, so do your homework and don't buy a car with the QR25DE engine!!!!!!!"