DTC P1721: Vehicle Speed Signal: Description
WARNING: This page is about a different car, the 2013 Infiniti FX50. However, it is still accessible from the selected car via links, so may be relevant.
The vehicle speed signal is transmitted from unified meter and A/C amp. to TCM by CAN communication line. The signal functions as an auxiliary device to the output speed sensor when it is malfunctioning. The TCM will then use the vehicle speed signal.