Thanks to both MG McAnicj and cgallen for answers. The problem has resovled itself, at least for now, but a quirk. My daughter finally took the car back to Firestone to address the blend door issue a couple of days before Christmas. Since the car is 9 years old, because NJ is supposed to have a cold winter, and because the car still had its original battery, I told her to have Firestone install a new battery. They did, but they installed it before looking at the blend door issue. Removing the old battery erased the error code history on the computer, so they could not read the error code when they installed the new battery. But, for reasons unknown, the whole HVAC system worked perfectly after that. The shop manager did some internet research and found something that indicated it may be nothing more than a faulty error code. He also found that if you push the automatic HVAC control buttons in a certain sequence, it will override the error code. That hasn't been tested yet since the system worked by disconnecting the old battery. If the blend door fails again, my daughter will try the button sequence and see if it erases the code. This car also has an erratic and faulty coolant system light (we have the coolant system flushed and changed at least once a year just to ensure there is not a problem). And, my 2000 Sable has an erratic and faulty brake warning light. There is no problem with the brake system, but the light never goes off. I'm beginning to think these cars produce false positive readings on various systems.