Pre-cat failure, that's what...especially on an '02. Do some searching on b15sentra.net and you'll find out why.
BTW, the gas mileage isn't bad at all depending on how you drive it. You can't look at the range due to the fact it has a small gas tank. If you're getting 250 to a tank, you're getting about 24-25mpg give or take. The most I've gotten range wise is about 315-325 going on a road trip cruising at 70mph the entire trip, getting about 29-30mpg. It depends on what type of driving you do and how you drive it.
And yes the Spec isn't the greatest car out there, but it's not as bad as people make it out to be.