No overdrive, automatic transmission

[BevSor]

I'm looking for troubleshooting help:
Any help or knowledge you have to share on this topic is appreciated.

The Problem:
This 2013 Yaris will not shift into 4th gear. It has an automatic transmission. High engine rpms at 60mph. No fault indicator. I don't have a tach, but it sounds like I'm heading toward 4000/rpms at 60mph.

My thoughts:
I believe the ECM makes a logical decision to change into overdrive, and activates a hydraulic valve that is controlled electronically to shift the car. I'm hoping one of the sensed conditions is stopping it from selecting 4th gear, and the transmission itself is mechanically fine.

Some Questions:
1) Does anyone know the logical conditions required for the ECM to select 4th gear?
2) I read on a previous post that there is a transmission temperature sensor that can prevent the ECM from selecting 4th gear. I'm hoping this is my problem. Does anyone know where this sensor is, and what the part number is? What are the wire colors going to it? What is the resistance of this sensor when it is cold, and when the transmission is warm?
Note: I went to a Toyota dealership asking for this sensor, and the guy couldn't find it in the parts breakdown. I ran into the same problem looking at their factory parts diagrams online.
3) If there is a hydraulic solenoid valve that is turned on electrically by the ECM to select 4th gear, does anyone know the location, part number and/or wires going to this solenoid? I'm thinking the valve could also be sticking mechanically, the electrical coil would look correct to the ECM, and not set a fault.
4) Do you have some other knowledge on this topic? If you know that I'm completely wrong looking at the temperature sensor or solenoid, please explain how it really works.

Thanks in advance for any comments. JP