I have a 200 GMC jimmy 4.3L. to make a long story short I replaced the fuel pump drove vehicle for a week then had the battery go out (low voltage yet started). At that time I started getting instrumentation errors (fuel,temp gauge inoperative, shift postion indicator not light, Air bag light on, seatbelt light on) I changed the battery and the instrumentation did not correct itself. I disconnected the Battery over night and now the car starts, runs for 5 to 10 sec. and dies. I attempted prior to this to have codes read at local auto store they said would not communicate. Based on that I went to bone yard and got a new ECM and installed it now all instrumentation appears fine however still starts and runs 5 to 10 sec. and dies. I don''t know what to try next. I do not have a code reader. Any help would be appreciated.

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2000 GMC