My 2010 G37x is having an issue.
It will not go over 3,500 RPMs - it will upshift and refuses to downshift.
It doesn't matter if I’m in sport mode or even try to shift it manually.
If I’m on the highway and accelerate it just stays in high gear and wont downshift if it would cause the engine to be running at over 3,500 RPMs.
Thoughts??

Mileage?
Modifications?
Check engine light on?

Look up the threads on the Hesitation and Lag issues. I had this happen on a 2009 G37x a TSB came out on it sometime back. I had it done - but got rid of the car not too long after. Seemed to fix it. But worth taking a look.

Here is a list of TSB's:

http://www.infinitig37.com/Infiniti-G37 ... itins.html

There are no TSB's which reflect a non-downshift issue, just harsh downshifts. Have you tried resetting the ECU, etc? I would check out the TCM (transmission control module)