We had a passenger in the back seat -- they commented on "cold
air" coming from the center vent.
Sure enough it was cold, despite the fact that we were toasty warm up front.
We turned up the heat/temp dials all the way. Tried "feet" "feet,
center, defrost" and "auto" modes for both driver &passenger
settings.. Always cold air out the back vent.
The only thing that seemed to affect it was the fan setting -- it did
blow more or less depending on the fan setting.
We rarely have passengers, so we never noticed it before.
I read that when the power goes out (change the battery) the heater
does a "self reset" kind of thing to reindex the stepper motors.
I did not know this when we put the new battery in - and I did
turn the car on & off rather quickly (just to verify the battery was
installed). So what I'm wondering is if this stepper motor is stuck in
one position or another -- or if there's something else to look for?
(dead stepper motor, connection/fuse) etc.