Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
That code is an indication that the trans controller has requesteed that the PCM turn the light on.
There are many things that can cause that, however, there has been a service bulletin on some that has to do with the diode in the TCM power circuit.
You might want to try getting a non biased shop to confirm that there are no issues with the (rebuilt) trans.
Here is what one of the bulletins states. See if yours may be in this one.
Date: FEBRUARY, 2003
Model: 2000-2002 ACCENT ATM
SubjectATM DIODE CORROSION IN THE CONTROL WIRING HARNESS
Some 2000-2002 MY Accent vehicles may experience ATM diode pin corrosion in the control wiring harness located behind the dash panel. Water from the cowl top panel may enter through an improperly sealed control wiring harness grommet in the dash panel.
A corroded ATM diode may result in the following symptoms:
If a vehicle with these symptoms is being serviced, follow the ATM diode replacement procedure provided in this TSB. Do not replace the automatic transaxle or control wiring harness.