Fill Line Union Check Valve
WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The fill line is equipped with a check valve. In the event of fill valve leakage, the check valve is designed to minimize the amount of CNG fuel leakage. The check valve must be installed with the flow direction arrow pointed toward the high pressure lock-off solenoid.