The blend door is electrically actuated, so vacuum doesn't play a role with it. Other vent doors on your vehicle do, but they wouldn't cause this problem.
Listen, my guess is if you retrieve the trouble code that is causing the check engine light to be on, you will find B1249. That is a code for the blend door actuator. I am very confident that either the actuator is bad, there is a poor connection to it, or there is no power getting to it.
When you say you checked the door, did you actually observe it moving when you changed from AC to heat?
Monday, November 3rd, 2014 AT 6:47 PM