more info from Rennlist's Bill Wagner:
"It seems to me that 1991 appears to be a changeover year, meaning that sometime in the 1991 production run the DMEs were changed (US only by the way) so that the diagnostic capability became useful. Some California cars had this feature enabled as well before it became a standard feature... Please keep in mind that for the most part the active Check Engine Light applies to 1991 and greater US MODELS ONLY"
http://members.rennlist.com/billwagner/CheckEngine.html
Quite helpful info.