Wondering if anyone else has encountered and solved this problem.
2004 G35 Sedan ~120K miles
Recent Transmission fluid change with synthetic fluid and anti-friction additive has not affected the following behavior
Car will frequently not shift into 5th gear when on the highway. RPM are consistent with 4th gear. Once this occurs, car will never shift into 5th gear until the car is restarted.
A P1716 code is being thrown when this occurs. The problem is detected when the transmission is in 4th, and perhaps trying to shift into 5th gear. When this occurs, the car will disable manual shift mode and switch to Drive "D" mode, and never allow it to use 5th gear again until the car is restarted. You are not allowed to use manual shift mode until the car is restarted.
The only way I have to get around the problem is to run the car in manual shift mode after restarting the car, and not allow the transmission to run in 4th gear for very long. If I manually shift quickly through 4th and into 5th the car drives fine in 5th gear forever on the highway. RPM are proper and consistent for 5th gear. However, if I allow the transmission to stay in 4th gear for very long, apparently the problem is detected and 5th gear and manual shift mode are disabled.
This happens consistently soon after reseting the code.