DTC P1456: EVAP Control System - Very Small Leak Or Does Not Operate Properly : Description
WARNING: This page is about a different car, the 2003 Nissan Pathfinder. However, it is still accessible from the selected car via links, so may be relevant.
This diagnosis detects very small leaks in the EVAP line between fuel tank and EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV), using pressure in the fuel tank. The EVAP canister vent control valve is closed to shut EVAP purge line. Vacuum cut valve by-pass valve will then open to clear line between fuel tank and EVAP-CPVCSV. EVAP control system can then monitor pressure inside of fuel tank.
If fuel tank pressure increases, ECM will check for leaks in line between the vacuum cut valve and EVAP-CPVCSV.