DTC P0840 Transmission Fluid Pressure Sensor A Circuit (Sec Pressure Sensor): Description

WARNING: This page is about a different car, the 2008 Nissan Maxima. However, it is still accessible from the selected car via links, so may be relevant.

The transmission fluid pressure sensor A (secondary pressure sensor) is included in the control valve assembly.
The transmission fluid pressure sensor A (secondary pressure sensor) detects secondary pressure of CVT and sends TCM the signal.