My guess it is their scanner, I had this same problem. If there is another autozone around go there and get a different scan, or go back to where you went and have them use a different scanner i believe most have two that they can use. there scanners are well used so the plug gets worn and wont work(on our cars
). Before you buy a scan tool, What is you goal with your car?
There are basically two types a code reader or a scan tool. If your car will always see stock parts and you just want the most basic abillity to read the stored codes get a code reader. If your really into fixing everything and or modding get a scan tool.
Does your car have manual controls on the AC or is it the automatic climate control?
My understanding is that you can set the auto system into a diagnostic mode with the correct series of button pushes/holds.