I have a 96 Altima that the check engine light is "on". It runs great and gets very good gas milage. What are the causes if this light coming "on"?

Thanks,
Paul

The light tells you that there is a trouble code stored in your computer. The code can be accessed with a scan tool. AutoZone and others will do it free of charge.
http://www.samarins.com/diagnose/checkengine.html.

Here are the possibilities that match the codes

http://www.obd-codes.com/trouble_codes/nissan/index.php

I have a 95 Altima and the check engine light came on about 3 weeks ago when I was backing down a dirt road with the parking brake on. Course I didn't remember I had it on and the rear wheels did spin most of the backing up but skidded for the rest. Car has been running fine with no obvious problems since then. Any idea why the light could have come on for that?

Yeah happened to me too, but my light urned off as soon as I took the handbrake off. If you didn't ride the brakes too long there should be no reason the light is still on. Anyway, I think you should get it checked out at autozone or discount auto parts for free.
Good Luck.