First off, identify your car by year, engine, and transmission. There are differences and without that information we can only guess.
Second, post the exact code number. Better yet, perform a SEARCH using the code as your search keyword to see if it's one that had been frequently brought up (sounds like it).
If you stiil need additional advice (like finding a new mechanic that understands what codes mean and how they relate to the CEL and the rest of your engine), don't hesitate to ask. Sounds like the poor SOB is batting 0 for 3 so far.
The light comes on then because the computer has decided that the operating conditions are met to perform a specific system monitor test. Certain of the monitor tests require steady-state highway driving (and a few other things) in order to execute.
Steve
