EVAP Canister Vent Restriction Test
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
If the EVAP vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The Powertrain Control Module (PCM) tests this by commanding the EVAP purge valve ON, open, and commanding the EVAP vent valve OFF, open, and monitoring the FTP sensor for an increase in vacuum. If an increase in vacuum is detected on 2 consecutive tests, DTC P0446 (a restriction in the EVAP vent system) will set.