Hyundai Repair: Accent trans shifting, overdrive switch, pulse generators

Question
I Have a 1999 Accent that will not shift into overdrive with in the first 10klm of driving & will down shift to 3rd without warning as well.Code p0722 was found , so both input & output sensors were replaced. no difference. tcm was replaced , no difference, Po712 trans oil temp voltage high code was found. On a scanner temp will fluctuate between -40 & 72 deg c. With the scanner reading -40, the transmission will still shift into 4th & stay there. ?

Answer
-40 C is the value that's displayed when there's an open circuit in the temperature sensor circuit.  This is often caused by a broken wire at the temperature sensor connector at the transmission, but can also occur from a defective sensor itself.  You should definitely locate and repair this problem.  If the transmission computer sees an incorrect temperature value without determining it's erroneous, it may not shift normally.

Has the P0722 code returned since replacing the pulse generators (input/output sensors)?  Has any repair caused the P0722 to not return?

The only things that should make the vehicle shift into third abnormally without setting a trouble code would be the temperature sensor issue you've described and a faulty overdrive switch on the shifter.