Do I press and hold the lock/unlock buttons at the same time ???
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Unfortunately these need to be reprogrammed at dealer...You could call around, maybe some other shops have scantool to reprogram this but not usually...Otherwise maybe you could try closest gm dealer in another town if possible. Let me know if you have questions, thanks.
You could try to disconnect battery for 10 minutes you can also try to press the lock/unlock button 10 times in row. Let me know, thanks.