If you're a do-it-yourselfer, buy an OBD2 (on board diagnostics tool) I
have an Innova 3100 ($150.00 - $200.00) Works great! All you do is plug it
into the vehicle's test connector located just under the driver's side dash
(in most vehicles). Turn the ignition to on (do not start the car)Press the
Link button, wait a few secs. Up comes an alpha numeric code on the screen.
You look the code up in the OBD2 manual or go to www.CodeReader.com to find
out what the code definition is.

It's probably a bad O2 sensor. Thats the only thing that's ever shown up
on my '98 Pathfinder. Find out what your local garage charges to do a
diagnostic test. The cost is about half of what an OBD2 code reader costs
in my neck of the woods. Consider getting one.  It pays for itself in no
time. Once the problem is fixed, you use the Innova to turn the light off.
Very easy to do! Changing an O2 sensor is easy too if that is the problem.

Good Luck!!!