Hello Mario
How are you doing? Before replacing the transmission or throttle body let's consider some things first. Since the stepper motor is new it's kinda odd that it will fail that easy which may also happen.
Possible Causes
- Faulty Step Motor
- Step Motor harness is open or shorted
- Step Motor circuit poor electrical connection
Check the "Possible Causes" listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector's pins. If still fails after the inspection then the throttle body and transmission must be physically checked and diagnose if it will require replacement.
Hope this helps and I'll be standing by for your response.
Thank you
Relly