Symptoms

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

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

When Detected

Vacuum cut valve bypass valve does not operate properly.

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. There are service bulletin for the P1491 code: P1491 Nissan Service Bulletin

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..