Problem with my 1998 Buick park ave
Mileage = 100,000
Engine size 3.8 6 cyl
GM tech = JK
question = Heater works normal for 15 minutes then blows cold air on one
side and hot air on other.

A common cause for this is a failed "temperature door actuator motor".There
is one for each side of the dash- driver side, passenger side. These motors
control the temp coming out of the vent. Have had a few of the heater - a/c
control units go bad, but not many. Is almost always the actuator motor.

2003 Buick Lesabre limited 3800 series.
Question = passenger side blows cold air - drivers side does not.  Both blow hot but passenger is
hotter.  Passenger door temperature adjustment button does not work.  Actuators are moving fine.
Answer: Usually, it's an actuator that is the cause of this
problem, but I'll assume they are ok since you say they are
moving fine. The next most likely cause would be the
control head, but that would be an expensive guess. You
could have a door problem inside the heater case, and you
would still see the actuator move normally. Depends which
route you want to go, but I'd be sure and check the
actuators again and possibly remove them to see if the
internal doors are broken before trying the control head.