DTC P1491: Vacuum Cut Valve By-Pass Valve - Does Not Operate Properly: Description
WARNING: This page is about a different car, the 2002 Nissan Pathfinder and 2002 Infiniti QX4. However, it is still accessible from the selected car via links, so may be relevant.
Vacuum cut valve and vacuum cut valve by-pass valve are installed in parallel on EVAP purge line between the fuel tank and the EVAP canister. The vacuum cut valve prevents intake manifold vacuum from being applied to fuel tank. Vacuum cut valve by-pass valve is a solenoid type valve and generally remains closed. It opens only for on board diagnosis. Vacuum cut valve by-pass valve responds to signals from the ECM. When the ECM sends an ON (ground) signal, valve is opened. Vacuum cut valve is then by-passed to apply intake manifold vacuum to fuel tank.