CNG Fill Valve O-Ring
WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
IMPORTANT:
Always perform this check prior to each CNG fuel fill.
Inspect the condition of the CNG fill valve O-ring. Replace the O-ring if it is damaged or missing.