01 GTP not shifting into overdrive

I have an 01 GTP and it will not shift into overdrive. The transmission is smooth in the other gears. Is there a problem with the torque converter or could it be something else not allowing it to upshift?

Welcome to AF.

One thing you can do is get it scanned with a scan tool capable of reading out transmission related DTCs. That way you can see if it is a solenoid at the valve body like a 3-4 shift solenoid. However, the problem of no OD (4th) can be mechanical due to a bad 4th spline which is a common problem in a 4T65E autoransaxle.

Some TCC related DTCs may disable OD.

I second the bad spline. My 2000 GTP's overdrive went out at around 80K. The spline was actually smooth as a babies bottom.
Hope this helps.