How to Use a Car Code Reader | eHow.com
This kinda walks you through the process. The code reader tool should also have the same steps to help you diagnose the problem. Check the owners manual for the location of the OBDII input jack on your car and follow the steps.
This 996 forum has little traffic for vehicles not in the air cooled category. You should also register a user ID on Rennlist and introduce yourself there and post your findings from the reader. It is a great site for 993 and 996/997 owners.
Good luck and hopefully you'll be able to re-set the check light and continue to drive many more trouble-free miles.