Reprogramming brain box for a 1996 Pontiac Grand Prix 3.1

A dead battery, or disconnecting the electrical system will not affect the ECM. If it won't keep a charge you likely need a new battery. Have it load tested to be sure before buying a new one though.

Tried that and still won't keep charge

So is the battery going dead while it’s parked or does it die while it’s running? Does it start and run ok when it’s boosted from another car? Have you made sure battery connection are tight and that all grounds are tight and clean? Have you had the charging system checked to make sure it’s working properly? Have you scanned the car for trouble codes? All of these things could help lead you to a solution.