I just got my 2002 Alero back from the shop for a security light issue and now the check engine light came on. How do you diagnose this problem if the car appears to be running fine?

welcome to AF! The check engine light indicates there is a problem with the engine emissions system caused by any number of factors. Take the car to an Autozone or Advance and ask them to scan it for you. They will give you the code. Once you get the code post it and someone will help you repair the problem.