anyone knowledgable about the ecu system?

hello everyone
just thought i'd throw out a question about my aerostar

1994 Aerostar (eddie bauer edition)
4.0Liter engine (EFI)
Digital dash display with full package

Now my question is does anyone know if the economy computer is built in the ecu? or is it a separate box? And if so? Where would it be located?

1st) Reason i ask is my economy display buggers out on me occasionally and reads 99.9 mpg on the "instant economy" and "average ecomony" display. I"ve tried disconnecting the negative cable on the battery for a few mins to try and reset it but to no avail it keeps doing the same thing.

2nd) What would it mean if the "check engine light" comes on during the EOKO test but when i check the engine codes it passes with all "111" or "11" codes which means it passes. Am i missing something here? A fluid level low somewheres? All checked ok far as i can tell.

I have more questions but i won't barrage the thread too badly at first...lets just do a couple at a time :)

Laterz,