Nissan Repair: odb code, nissan sentra xe, canister purge valve

Question
A friend hooked up an odb reader to my 2001 Nissan Sentra XE, with a 2.0 L engine and a 5 speed manual transmission, because the service engine soon light came on.  The code that it gave was P1491.  According to his manual for the odb reader, this is a manufacturer specific code.  Are you able to tell me what this code means?

Answer
Faye,

The vacuum cut valve bypass valve is a solenoid type valve and generally remains closed. It opens only for on board diagnosis. The vacuum cut valve bypass valve responds to signals from the ECM. When the ECM sends an ON (ground) signal, the valve is opened. The vacuum cut valve is then bypassed to apply intake manifold vacuum to the fuel tank.  I would replace the canister purge valve.