Start by checking the "Possible Causes" listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector's pins.
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.
There is a factory service bulletin for all 1998-2002 Nissan models.
1998-2002 Nissan Factory Service Bulletin OBDII Code P1491
The cost to diagnose the P1491 Nissan code is 1.0 hour of labor. The auto repair's diagnosis time and labor rates vary by location, vehicle's make and model, and even your engine type. Most auto repair shops charge between $75 and $150 per hour.
Vacuum cut valve bypass valve does not operate properly.
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.
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