Why won't my car drive in Overdrive?

I would suspect it is a bad tranny or they put a 3spd in you 4spd car, or something got streched

I have the same problem with mine. The overdrive is the 4th gear and seems to fail first. You have a choice: You can live with it or you can spend around $2000 to rebuild the tranny. There is kind of a third choice. You can change the fluid and filter and see if that helps. If you change the fluid make sure the fluid is replaced with fluid that meets the ATF+4 spec (This is for Chrysler, Mitsubishi and I think Hyundai trannys). If you live with it you gas milage will never be as good again, but it will be OK.

This question is old but just in case ... OD on these transmissions is 4th gear. The clutch that controls the 3-4 shift can fail either due to heavy use or low fluid and when it does, you don't get 4th gear -- engine just revs unless you lock out OD. (OD lockout button on shift lever.) When you lock out OD, then orange warning light should come on on dash but an ethically challenged mechanic might disable the lamp. The clutch is not a terribly expensive repair -- $100 part and can be done without removing the transmission. However it takes quite a bit of skill. You might find a good transmission man who would do it for $400-500. Or contact me -- I might get interested in buying the car, at the right price!

Yes, I like Walt_Hutchenses" scientific approach to the problem-literally from more simple to more complicated. This is most easy to check OD light bulb, then workability of OD button on a shifter, then wiring to avoid advancing toward disassemble entire transaxle, to live somewhere behind you a peddling disckrepansy.

Some Mitsubishi transmissions have a solenoid on the tranny case side that activates overdrive. Seems fairly simple to me if this were the case with yours to get that solenoid and replace it yourself or have the shop fix it. Not sure if this applies to your car. The wiring to the solenoid could be the issue if it does apply. Just a thought.