What would cause the vehicle to tell itself that it doesn't need fuel?

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When our trailblazer would not start we determined that it needed a new fuel pump. We replaced the fuel pump and checked the fuel relay but it will still not start. Before we changed the fuel pump it started sporadically. The gauges went crazy and the vehicle quit when I was driving it and then would not restart. We are stumped! The device we plugged into it to see what was wrong just said that the it was telling itself that it didn't need fuel. HELP