Hello, and thank you for choosing Just Answer!
You are WISE for wanting to test before replacing. You really need a scantool for this one, though.
Here is everything Nissan has on the P1491 on your Altima.
COMPONENT 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.
DIAGNOSTIC TROUBLE CODE CONFIRMATION PROCEDURE CAUTION: Always drive vehicle at a safe speed.NOTE: If "DIAGNOSTIC TROUBLE CODE CONFIRMATION PROCEDURE" has been previously conducted, always turn ignition switch "OFF " and wait at least 5 seconds before conducting the next test.TESTING CONDITION:
Before performing the following procedure, confirm that battery voltage is more than 11 V .With Consult
OVERALL FUNCTION CHECK Without Consult
DIAGNOSTIC PROCEDURE COMPONENT INSPECTION Vacuum cut valve bypass valve Check air passage continuity
With Consult Perform "VC/V BYPASS/V" in "ACTIVE TEST" mode.OR
Without Consult If NG or operation takes more than 1 second , replace vacuum cut valve bypass valve.
Vacuum cut valve Check vacuum cut valve as follows: