a sensor would not be under powertrain but whole throttle body would be. i don't know how if the sensor and TB is all one unit or you can get it separate.

 

I had the same issue and they said it was the throttle position sensor. They replaced the throttle body to fix the issue, so I would assume that replacing the throttle body is the standard operation at the dealership when it comes to throttle position sensor issues. My G35x is still covered under both Bumper to bumper and the powertrain warranty and I am not sure which one they stated that the throttle body was covered by.

 

The TB and sensor are all one unit. I had mine replaced under warranty.