Circuit Description

During specific operating conditions, ECM performs various tests on Evaporative Emission (EVAP) system. System tests consist of the following series of events.

After ensuring that EVAP purge solenoid valve duty cycle has dropped to zero percent, indicating that valve is closed, ECM commands EVAP canister vent valve closed, sealing system. ECM monitors accumulation of vapor pressure within fuel tank via fuel tank pressure sensor. EVAP test is aborted if vapor pressure is too high. If any vapor pressure is measured, it will be used later to compensate a pressure reading during small leak detection test. If a vacuum is measured which exceeds a calculated limit during vapor accumulation, this DTC will set. EVAP canister purge solenoid valve is then opened. Simultaneously, vent is opened.

If vapor pressure does not bleed off or bleeds off too slowly, this DTC is set. Once EVAP purge solenoid valve reaches its desired position for remainder of diagnostic test, vent valve will be closed again. This causes vacuum to be applied to entire EVAP system. ECM monitors vacuum level within system. If desired vacuum level cannot be achieved, or if vacuum level is reached but took too much time, DTC P0455 will set. Once desired vacuum level is reached, purge solenoid is closed, sealing system. ECM continues to monitor fuel tank pressure sensor signal, measuring rate of vacuum decay. If system holds vacuum, vent valve is opened and test is completed. In addition to system tests, ECM monitors circuit integrity of purge solenoid (DTC P0443), vent valve (DTC P0446) and fuel tank pressure sensor (DTC P0450).

Conditions required to set DTC are:

• Diagnostic test has run to completion and determined that this DTC was detected.