Component Functional Description - OBD Vent Valve

Functional Operation: 

The normally closed OBD Vent Valve is used to provide open venting to the atmosphere when energized by the Powertrain Control Module (PCM). The valve is used as part of the Evaporative System diagnostics on Turbocharged engines for two purposes:

1. ESIM Switch Stuck Closed Diagnostic:  The evaporative system must be vented to atmosphere to relieve the vacuum and open the ESIM switch to perform the small leak diagnostic. Due to the check valves in the Proportional Purge Solenoid (PPS) on turbocharged engines, the OBD Vent Valve is energized by the PCM to vent the vacuum from the system. When the system is being actively vented the ESIM Switch Stuck Closed Diagnostic is used to determine that the ESIM switch is functioning properly.
2. Purge Performance Diagnostic:  This diagnostic looks for a delta pressure change in the Fuel Tank Pressure (FTP) Sensor signal when the Purge Solenoid is energized. If vacuum is already trapped in the system it may be difficult to generate enough of a delta pressure change to obtain good separation between a passing and failing system. Since both sides of the system are tested during each drive cycle, the OBD Vent Valve is energized to eliminate the excess vacuum before the Purge Flow Monitor Diagnostic begins for each side.

Diagnostic Overview:  The PCM performs open circuit and short circuit diagnostics on the control circuitry for the OBD Vent Valve. Detecting a valve that is mechanically stuck open or closed is caught by other EVAP system diagnostics. A stuck open valve would cause a system leak and would fail the small leak or large leak diagnostic. A valve that is stuck closed would cause one or both of the ESIM Switch Stuck Closed or Purge Performance diagnostics to fail.