Mitsubishi Repair: 3000GT SL only drives on 3rd gear, transmission control unit, highway speed

Question
car: 1992 3000GT SL automatic non turbo FWD. 87*** miles.
issues: car runs smoothly but only on third gear.
do i have to replace the complete transmission!

thanks in advance

Answer
Muhammad,
This sounds like your transmission is stuck in "limp mode".  This can happen when your transmission control unit detects a problem with the transmission and holds it in 3rd gear to prevent further damage, while still allowing the car to be driven at highway speed.  I would recommend bringing the car to a transmission shop for further diagnosis.  You may not have to replace the transmission, but only they would be able to determine that.
Good luck!