The "service engine soon" light came on my 2001 nissan
Altima without any warning. The car drives great. What
Can be causing the light to come on?

Niece,

The Service Engine Soon light detects a problem in the emission system. If the light stays on, you'll be OK until you can get it on an analyzer. If it flashes, don't drive it any longer than absolutely necessary or it could damage some of emissions system. I recently had the same problem combined with difficulty putting fuel in the tank. If you're having the same thing, maybe I can help you.

My problem was a valve that connects to a small box by the gas tank (Nissan part #14935-54U04). It cost me $37 and took about 15 minutes to replace on my 1998 Altima. Under the car, behind the gas tank there's a plastic box held on by 3 small 12mm bolts. Take out the bolts, slide off the 3 hoses and the 1 electrical connector in the front of the box and take the box down. Take off the valve that's held on by 2 phillips head screws, put on the new one, and bolt it back up under the car again and you're back in business. When you take the box down there is a little pin that goes through a front hanger and another in the back that rests on a bracket. Just make sure you put the box back in the same way. It's really easy to do and the dealer will ding you for $140+ if you have them do it. The only problem is that I don't know how to reset the engine light, but if worse comes to worse and I can't figure it out, I guess I'll pay the $20 a local guy charges to reste the light.

Hope this helps!

Thanks so much, you've been a great help.

The service engine soon light came on in my 2001 Nissan Altima and when I took it in to the dealer, they said that there was a very small evaporative leak and that I needed a new fuel tank that cost $900!! After reading messages on this website, I think that maybe it just needs the evaporative control valve because I have noticed that when filling it with gas, the nozzle keeps clicking off. How can I find out if this is what the problem really is?

The 2001 Altima 3.0 has also 4 oxigene sensors that had gone bad several times on my Nissan after the car had 35,000 miles, if you take it to the dealer they will charge you around $1000.00 to replace .
The cost of the sensors is $100.00 in most auto part stores and not that hard to replace.
With an electronic analyzer that coast around $60 you will be able to read the code that will direct you to the faulty sensor.