DTC P0456: EVAP Control System Very Small Leak (Negative Pressure Check) Or DTC P1456: EVAP Control System Very Small Leak (Positive Pressure Check): Description
WARNING: This page is about a different car, the 2002 Nissan Xterra and 2002 Nissan Frontier. However, it is still accessible from the selected car via links, so may be relevant.
This diagnosis detects very small leaks in the EVAP purge line between the fuel tank and EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV) using engine intake manifold vacuum. If ECM detects a very small leak, DTC P0456 will set. If ECM detects a small leak, DTC P0442 will set.