Possible causes

– Vacuum cut valve bypass valve
– Vacuum cut valve
– Bypass hoses for clogging
– EVAP control system pressure sensor and circuit
– EVAP canister vent control valve
– Hose between fuel tank and vacuum cut valve clogged
– Hose between vacuum cut valve and EVAP canister clogged
– EVAP purge port of fuel tank for clogging

Tech notes

If the EVAP canister is OK, and there are no other visual damage to the EVAP system (e.g. loose or broken hoses), replacing the vacuum cut valve bypass valve takes care of the problem.

When is the code detected?

Vacuum cut valve bypass valve does not operate properly.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P1491 NISSAN SENTRA Description

The vacuum cut valve and vacuum cut valve bypass valve are installed in parallel on the EVAP purge line between the fuel tank and the EVAP canister.
The vacuum cut valve prevents the intake manifold vacuum from being applied to the fuel tank. 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.
2001 Nissan Sentra Vacuum cut valve bypass valve location:

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NISSAN Technical Service Bulletins for P1491 code