DTC P1813: 4WD Shift Switch: Description
WARNING: This page is about a different car, the 2012 Nissan Pathfinder. However, it is still accessible from the selected car via links, so may be relevant.
The 4WD shift switch allows the driver to select 2WD or 4WD and 4H or 4LO. DTC P1813 will set if more than two switch inputs are simultaneously detected by the transfer control unit due to a short circuit in the 4WD shift switch.