Horn slip ring/rotary coupler

How do you know when the horn slip ring /rotary coupler is u/s? Do they gradually give up or just stop working? A friends TF's horn stopped working and at the same time the airbag light came on. I seem to remember when I was looking at TF's I came across a car with the same symptoms and the dealer said it's the slip ring and it was an easy change for him.

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