Being a cheapskate, and bored, I decided to check it out and see about fixing it.
Anyway, I started taking pieces out, and testing them. One of the pieces I took out and tested was the little servo that actuates the "fresh vs. recirc" door. Here's a picture of it with the cover off:
There are about 200 gears in there, allowing a tiny motor to turn a little arm on the outside exactly 90 degrees when it's turned "on", and another 90 degrees when you turn it "off". I tested it with a 9V battery, and found that it works fine. So did the wires that connect it.
Another thing I took apart was the HVAC panel itself, to test the switch.
I'm usually puzzled about why cars cost so much. Looking into things like this gives me an idea of where some of the cost comes from. This panel is an intricate piece of engineering, and it's just the little panel that runs the heater and A/C. It's the kind of thing that, when you look at how it works, makes you go "Cool!".
For example, the little buttons on the panel all light up when you turn the lights on. But instead of each of them having a separate light bulb, there's a 7-fingered clear plastic "hand" attached to the circuit board behind the buttons. It sits over two tiny light bulbs on the PCB and carries light from them up to the buttons on the panel. The ends of the "fingers" have a rough texture, which diffuses the light in that spot behind the little icons on the buttons. There's a whole assembly consisting of little springs, plates, hooks, and sliding wedges, that all exist to make one button pop out when a different one on the same panel is pushed. It looks like some kind of amusement park for fleas.
I think it's cool that these intricate little parts are made by the thousands, and every one of them is generally able to cope with the bumps and thumps of everyday life for many years. I don't know if they're made by robots, or people in lab coats, but whatever it is, there's a pretty impressive process in place somewhere that makes it possible.
Anyway, I tested the switch, the motor, the wiring connecting the two, and they all were fine. The culprit: There's a snap-in connector on the back of the HVAC panel that wasn't snapped into place. It basically "fell out" of its hole when I pulled the panel loose. Two of the pins on the end of that connector are the ones connected to the servo. The one that turns the motor "on" was fine, but the one that delivers the "off" signal to it was apparently disconnected. It's snapped in now, and it works. It's somewhat satisfying to push the "recirc" button and watch the little servo going "whirrrrr", opening and closing the door like it's supposed to.
Here's the bad part: I spent probably 3 hours jacking with this thing, and I probably wore out a few million brain cells. All for the ability to push a button and get fresh air! Talk about "high effort, low reward"... Oh well, it's still more fun than corporate IT.