The check engine light on my 1999 2.5 RS came on a few days ago. I did some searching and found this description of the Subaru system.
formatting link
(Yahoo answers) The possible time delay between the actual failure and the light coming on is one thing that got my attention. Add intermittent failures to that. It's no wonder mechanics have trouble diagnosing problems at times. Even with all the modern diagnostics the troubleshooting could come down to an experienced guess.