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You might try re-training it. If memory serves me, I think you hold both the lock and unlock buttons down at the same time for 3 seconds and then the horn honks. If I am wrong, someone will correct me.
Yes if the retraining doesn't work you'll have to get new ones.
I got mine from GMPARTSHOUSE.com for $90 each and I was lucky because they are here in Phila so I just drove over and got them. They do go bad...what you could do is take it and have it tested at Auto Zone or the dealer for free as well