Cooler Check Valve
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Located in either the case cover or cooler line fitting depending on model. Allows cooler fluid to pass through the cooler and provide lubrication for the transmission. It also prevents converter drainback when the engine is off.