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I have a 03 Z71 with 76k mi. Around 40k miles I heard a clicking noise coming from under the dash and dismissed it for quite some time. Then after a while I noticed if I took the vent control knob and turned it one click back from full top vents, the clicking noise went away. So I just left it there and drove for some time (a couple of years now). About a week ago I was leaving work and started the truck turned on the A/C full blast and I couldn't get the top vents to blow air. It was like the control gate would only go from floor vents to de-frost vents, no top vents. It's an intermittent problem but I believe it has to do with either the control box where the knob is or a relay on the gate has a bad solder joint. Which may be what the clicking noise is.

Anybody have this issue before? How did you fix it?