HVAC temperature controls
 

New guy here with a 2000 Z71. When I select the red part of temp control it blows hot air but when I select the blue area it still blows hot air. The puzzling part is if I shut the truck down for a few minutes and select the blue area before I start the truck it blows cool air until I select the red area then the process starts over. The blower has different speeds and I can select different zones. Any help would be appreciated.