It could be low on freon, but if I remember right I had this problem with the Avalanche once. The conditions had to be just right for it to happen. For instance, it will do the same thing, if you push the button for recirc when the vent selector is either on the foot well, or defrost. It will blink, and go back to fresh air option. Where are you that you need AC right now? If it's cold outside, it may have an a temp sensor that won't allow it to run if the temp is below a certain point. I'll have to check on this.
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