Hi: I'm an owner, not a mechanic but I know the answer! Make sure all accessories are off, don't open or close the door or do anything while you are doing this: Turn key to ACC position and watch the security light flash for 8 minutes. Once it stops flashing, turn key to off, then turn on to start. It should reset the security system for now. Mine does this every few weeks, then won't do it for weeks, then do it again. There's a kit to bypass the system available through GM but its $300 or so. Good luck.
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