Actually, I think you are looking for a vacuum leak. The vents are actuated by vacuum. If vacuum is weak, the heat amy not make it to the vehicle. Also, there is a blend door that will do the same thing. It's located near the heater core. Check under the dash for vacuum leaks as well as under the hood from the soruce. With the engine running, you may hear a hissing.
Let me know what you find.
Tuesday, December 22nd, 2009 AT 7:09 AM