Any idea why the mix door actuator turns back to cold when I select defrost. The heater makes good heat on the first three selections but as I turn it to mixed defrost and full defrost I can watch the mix actuator turn back to the cold position. There is a air recirculation button on my control panel but switching it on or off makes no difference. (man has this heater been a pain in the butt) glad my rear heat works good. Dave