Nissan Repair: Service engine soon error, sentra 4dr, engine management system

Question
I have a 2001 Nissan sentra, 4dr sedan XE
On the dask board display the message: "Service engine soon"
What is the problem and how to clear it out?
Note that the car still run good, I'm keeping a good care for oil change, etc...
Thanks.

Answer
Doc,

The Service Engine Soon light means that there is something throwing a code on the computerized engine management system.  You have two choice.  The first is buy a code reader for about $65 bucks and read the codes yourself.  You can get one of these from Autozone, Checker, Pep Boys or the like.  The other option is you can take it to Autozone and get them to read the code for free.  That will tell you what is going wrong with the system.