I have a 01 cummins and love it, The problem is it stays in first gear until i let off on the pedal then it up-shifts. Sometimes it will downshift to first then after i again let off on the fuel it shifts back. all other gears shift normal once it gets to speed (past 35 mph) No noise from the tranny, fluid is clean, and no dtc codes at this time. could it be the pressure sensor and/or 1-2 solenoid? let me know Thanks you guys

 

First thing to check is all your grounds. Batteries, chassis, engine, etc. This is a common cause.

Governor solenoid and solenoid pressure sensor would be the next item that will do this.

Checking / cleaning the grounds doesn't cost a dime, so that would be the first place to start.