WARNING: This page is about a different car, the 2018 Nissan NV3500 HD, 2018 Nissan NV2500 HD, and 2018 Nissan NV1500. However, it is still accessible from the selected car via links, so may be relevant.
The EVAP control system pressure sensor detects pressure in the purge line. The sensor output voltage to the ECM increases as pressure increases.