Geez... well assuming it's the same as the Xterra code-wise it's a failure to communicate between the transmission control module and the ECM. The service manual just tells you to check for wiring harness problems between the two.

an interesting quote:
"The malfunction information related to A/T (Automatic Transmission) is transferred through the line (circuit)
from TCM (Transmission control module) to ECM. Therefore, be sure to erase the malfunction information such
as DTC not only in TCM (Transmission control module) but also ECM after the A/T related repair."