Archived Trans 1st to 2nd shift problem

[b_monty1]

I have a 2000 CTD with auto trans that shifts fine when cold, after it warms up it seems to stick on the 1st to 2nd shift for the first time (about 2100 RPM) but now it has gotten worse, doesn't want to shift out of first gear at all after truck is warm but shifts fine when truck is cold.



any help is appreciated.

 

[b_monty1]

I checked for codes and got this



P0720

Low Output Speed Sensor RPM Above 15 M. P. H. or



Output Shaft Speed Sensor Circuit



does anyone know what it is??