I just bought a 2001 Cavalier running on gasoline and went to fill it with CNG for the first time. It wouldn't take any fuel. I thought it was a problem with the pump but got the same result on 3 different hoses. I decided that the tank must be full but the gauge is showing empty. As I was leaving the pumps it actually started on CNG and the fuel gauge shot up to almost 3/4 full and I drove about 50 feet and the gauge went back to empty and it switched to gasoline and is showing empty on CNG again. Try as I might I cannot get the red light on the switch to go out and there is no pressure in the line. I have a "donor car" that I can pull parts from to test out any problems. Where should I start?

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