DTC P1801, P1811: Power Supply Circuit For Transfer Control Unit: 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 transfer control unit controls the transfer control device which controls shifts between 4H and 4LO and between 2WD and 4WD. When the vehicle battery is removed, the power supply to the transfer control unit is interrupted, and self-diagnosis memory function is suspended. These DTCs may also set when the power supply voltage for the transfer control unit is abnormally low while driving.