Hyundai Repair: 2001 elantra GLS A/T shift problem, shift problem, shift cable

Question
I just bought this car and it was the smoothest shifting car I've ever owned. Then while I was traveling at about 60 mph the transmission dropped into 2nd or 3rd gear (18 to 23 mph) and would not shift. After driving at that speed for a while it would shift again then it would slam back down again after 20 miles or so. I replaced the input and output sensors with the ones I took out of a 2001 sonata (with a blown motor). The sensors were fairly new. Now it doesn't slam down into gear but it shifts hard and sometimes wont shift into 3rd gear. If I shit to neutral and back to drive it will shift. Also it appears to have the same trans as the sonata except there is no screw on filter (there is a place for it). Is this normal? Also how do you re-learn the tcm?

Answer
I'd recommend checking diagnostic trouble codes first.  There's a significant possibility this is an electrical/sensor issue.  

If you get no trouble codes, I'd recommend taking the car to a dealer for diagnosis.  Since you say shifting to neutral and back to drive solves the issue, I suspect there may be a problem with the adjustment of the shift cable or with the range switch itself.  The technician at the dealer can monitor on his scan tool the readings returned to the TCM from the range switch.

You can re-learn the adaptive values by disconnecting the battery for 10 minutes and then driving the vehicle for at least 20 minutes, being sure to have the transmission shift through all the gears several times when at operating temperature.