Originally found this here. I've never tried it and make no claims to its authenticity.

1998 Plymouth Caravan Diagnostic Codes

I just found out that for the 1998 model of Caravan/Voyager to get the diagnostic codes you have to do the following:
  1. With the ignition key to the off position, press the trip/reset keys on the odomoter simultaneously and while holding it switch ingition key to on and hold on to the trip/reset keys for 5 more seconds.
  2. The odometer displays CHECK 1 and CHECK 2 while the diagnostic system is retrieving stored codes.
  3. If no problem, the odometer reading comes back. If problem the error code is displayed.