I think is problem is worth sharing.
While removing the intake plenum on a 2003 i35 infinity to replace the rear plugs, the tech noticed that the throttle body was heavily carboned-up. The tech cleaned the throttle body. After he reinstalled the plenum, he noticed that the idle would not come down below 1000rpm and after 5 minutes the check engine light came on logging an error code for RMP out of range.
After unsuccessfully attempting to perform an idle relearn through the scanner, we concluded that the throttle body had failed. We researched the problem and found a lot of know issues with this system. Apparently, throughout the years, the carbon build up closes around the throttle plate reducing the normal air flow and the computer makes corrections. By cleaning the throttle plate, now the idle is too high and for some reason the computer cannot bring down the idle.
The fix: replace the throttle body motor and perform idle relearn.
Try explaining this to the customer!