I have owned my 2007 G35x sedan for 2 year and haven't had a single problem until a few days ago when the car basically stopped shifting after a pull. when I first start the car it drives good but once I speed up it shifts up and I believe it is locking in forth gear when trying to shift to 5th. I drove all they way to work the other day without it shifting once. I talked to some people and we all agreed it was probably the valve body. I then took it to a shop to get it looked at and they said that my transmission is mechanically good and that it is an electric problem. I don't fully understand what's happening but from my understanding the ecu thinks its shifting but the tcu isn't responding.

the car can still drive, struggles to start moving because it doesn't have enough torque in 4th. Manuel mode doesn't shift the car but my screen shows that its shifting.

codes:
P1815 - M-Mode Switch
P1757 - FR Brake Solenoid
P0735 - 5GR Incorrect Ratio
U1300 - AV Comm Circuit
U121F - Internal Comm

 

These ECMs are wacky, mine is also not in it's proper state but first thing to do is to erase the current codes and see which one(s) come up right away. Those will be the ones to deal with first. When my rear left ABS sensor went out, my trans goes into *limp* mode, SLIP comes on and it shift SUPER HARD for some reason. After i erase the code(s), everything is fine.

You can also check the shifter switches if they're sticking.
Heres a link i found: P1815