DTC P1815: Manual Mode Switch: Description
WARNING: This page is about a different car, the 2005 Infiniti G35. However, it is still accessible from the selected car via links, so may be relevant.
Manual mode switch is installed in A/T device. It sends manual mode switch, shift-up and shift-down switch signals to TCM.
TCM sends the switch signals to combination meter. By CAN communication line. Then manual mode switch position is indicated on the A/T position indicator. For inspection, refer to A/T INDICATOR CIRCUIT .