Canister Vent Restriction Test
WARNING: This page is about a different car, the 2004 Chevrolet Corvette. 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 control module tests this by commanding the EVAP purge solenoid ON, commanding the EVAP vent solenoid OFF, and monitoring the FTP sensor for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will set.