my vehicle has automatic transmission and will not shift into overdrive.

There is a diagnostic box on the passenger side fender under the hood...its labeled on top of the box. Take a paper clip and make it into a U shape and put it into the TE1 and E1 holes...these slots are labeled under the cap in a diagram. Then get in and turn the ignition switch past ACC to on and watch the check engine light blink. The codes will cycle through and start all over so have a pen and paper handy.