Car starts but dies right away. New Key, tried a different ignition switch. on 2002 Saturn SC2

My compass shows voltage. The car starts, then the voltage goes to 7vdc then to 0vdc and dies. The car runs good, the engine starts right away but dies in a few seconds. I wonder if it is the computer or the security system. The car worked fine for a couple of days then it came back again. Since it is not the ignition switch what could it be?

Asked by for the 2002 Saturn SC2