Can anybody help me .. I have 99 Sentra SE.. Service Engine Soon light came on.

What could be the problem..??

Well, if you don't feel anything wrong with the car, it's probably an emissions problem. Like the oxygen sensor or something.

What mileage are you at?

i think i have an emissions problem, then the light went away after i gave hard throttle...it makes no sence, but now it doesn't stink anymore...?

it has got 87,000 miles..

maybe a sensor was malfunctioning? I can't think of a reason for hard throttle fixing an actual problem.

You might wanna get it checked out to stay on the safe side.

Mileage 89K

I changed the O2 sensor (Before Catalytic Converter).. Drove for 300 miles..

Light came back again..
Mechanic says : need to change "After Catalytic Converter"..

Will go tomorrow...