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Published on Apr 13, 2011
For quite some time, my truckling has been suffering from a vacuum leak that would sometimes make the climate controls not work (especially under hard acceleration--needed to keep after the Keykeeper's supercharged Pontiac!). Also, when the engine was off, airflow would revert to the dash vents due to vacuum loss. (GM vehicles have a failsafe position that is a mix between the heater outlet and defroster vent. Why mine went to the dash vents I don't know.)
If you are not able to switch between HOT and COLD airflow, your blend door actuator has probably failed. On a 2003 truck and probably anything 98-up, the blend door is operated by an electric motor. Failures are usually due to one of the gears stripping out or cracking. If you're faced with this repair, you may find this video from Youtuber Steve Crosier worthwhile: https://www.youtube.com/watch?v=WiwZe...
A number of people have indicated that the vacuum line going down into the fender well area connects to a vacuum reservoir. I never had to troubleshoot that and can't speak to where it goes.
As of April 2013, I don't own this truck any longer. Anyone seeking detailed service advice about this or any other repair should get the GENUINE GM FACTORY service manual. Do NOT waste your time or money on the Haynes or Chilton's manuals -- they are just about worthless.