DTC P0446: EVAP Vent System Performance: Description

WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

Evaporative Emission (EVAP) system is used to store fuel vapors as to reduce amount of unburned fuel from escaping into the atmosphere. EVAP system consists of EVAP canister, fuel tank pressure sensor, EVAP lines and hoses, EVAP purge solenoid (normally closed), EVAP service port, EVAP vent solenoid (normally open), fuel tank and ECM. ECM monitors EVAP system for circuit faults in fuel tank pressure sensor, EVAP purge solenoid and EVAP vent solenoid circuits. ECM also monitors EVAP system for large and small leaks. During DTC diagnostic, EVAP purge solenoid is commanded to a fixed duty cycle (pulses valve) and EVAP vent solenoid is commanded on (closed) in order to build vacuum. Once vacuum is obtained, EVAP vent solenoid is commanded off and vacuum should decrease rapidly. DTC sets when vacuum decay is too slow, indicating a blocked vent system.