the air or heat only blows out of the defrost no matter what control is set on. on 2004 GMC Yukon Denali

problem has occured for the past 2 weeks.

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4 answers
You have lost your vacuum to the control head. You most likely have a bad control head. Defrost is controlled by vacuum not a blend door.
all three are good answers
Vacuum leak under hood pass. side near firewall.
maybe your blend door is broken