Heater starts blowing cold air on passenger side?

Hi, I'm new here to AF (been browsing for a while though). I have a 2004 Trailblazer LS with about 120k on it. When I start it and it warms up the heat works great on both sides. After about ten minutes I can hear the door that separates the hot and cold air close (or open not sure) and ice cold air starts blowing out. I tried to change the blower motor resistor hoping it was just be bad power going out... No dice. I'm stuck, I have heard a ton about vacuum lines but have no idea where to start. If anyone can help that would be outstanding!

The actuators (there are five under the dash on the trailblazer platform) are all electrically powered. No vacuum any more. Sounds like your passenger side temp actuator gave out. Not unexpected. They have a terrible failure rate, especially as the vehicles age and you change the battery (which forces them to recalibrate and a lot of weak ones die then).

http://www.roadie.org/allactuators.jpg

Roadie, nicely done and spot on info!

Thanks Roadie! I was able to pull the glove box door down and find the actuator right away. Sure enough I turned the key to ACC and moved the temp selector up to heat, and I watched the actuator drive CCW. Moved the temp to cold, it drove CW no problems... I moved the selector back to heat and waited. After a few mins the actuator drove back to the cold position on it's own. Lucky for me I pulled the connector off and cleaned the pins of both the connector and plug, that seems to have left me problem free! Cause even the OE replacements are like $140. To think I spent an entire winter freezing, kinda funny though now that spring is here. None the less I am greatful for your help.