Find a facility with a scan tool that can access the Body Control Module. They will need to look at the data. Ideally the scan tool will have bi-directional control. You should be able to watch the horn be pressed via the scan tool in the Body Control Module. If you can see it is pressed, but no horn sound, try to operate the horn via the scan tool. GM supports this functionality test. If the horn does not operate, check the horn relay. It could be a broken wire between the BCM and the fuse box, or between the horn contact switch and the BCM.
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