This is my 4th post after 10 months on this matter, if you include Reddit. After some time driving, whether it's 5 minutes or 30 minutes, the transmission gets stuck in neutral. The engine still spins but cannot shift. The conductor plate within the valve body has been replaced, which we believed was the cause, but it was not. I've been researching with my dad, who is a great mechanic, and he doesn't understand the issue either. The last thing I'd want to do is take it to a repair shop, especially since my dad has all the tools to repair the vehicle, plus many repair garages scam you anyway.

I drive a 2004 automatic. The transmission name is NAG-1 if that is important.
- Error code P0715: Invalid RPM/invalid engine turbine speed.
This was believed to be caused by the N1/N2 sensors on the conductor plate, but now it could be bad wiring or another bad sensor somewhere else.
- Error code P0730: Invalid gear ratio.
This is likely caused by the previous error since the car doesn't know when to shift if it doesn't know how fast the engine is turning.

Possible causes:
1. TCM error not cleared.
I'm currently looking into this, but I don't think I have an OBD tool that can do that.
2. Electrical failure inside or around the TCM.
Checking the transmission connections shows no fluid leaking or wicking up the wiring to the TCM. Maybe the TCM has a bad connection, or another sensor has failed.
3. The link between the mechanical parts and the TCM is bad.
It could be that a sensor or wiring outside of the transmission housing is bad. It could be because of overheating if the transmission dies mid-operation. This is a shot in the dark.

I'm not a mechanic, but if my mechanic father doesn't understand this issue, then it could be a huge and expensive repair. I've been doing a lot of research online to get to the bottom of this, and I should be able to repair it myself since I have all the tools to take the entire car apart.
If anyone has had a similar problem solved, knows how to repair, or has suggestions on what to do next, please let me know. I appreciate any comments or suggestions.

Thanks for reading.