Something useful that I found... if you have a Thomas bus with in-dash diagnostics,
here is a list of the fault codes to help you run down an issue that triggers a check engine light. Hopefully it will be of some help to some of you.... so far as I know, any model year that any of us are likely to have is covered by this document.
I found this out of necessity, I have a check engine light that comes on after the bus has been driven for a while, but is back off the next time I start it (once the engine cools down). Guess I will need to take it out, drive it until the light comes on, then bring it back and go through the code recovery process (also detailed in this document).