problem has occured for the past 2 weeks.
the air or heat only blows out of the defrost no matter what control is set on. on 2004 GMC Yukon Denali
all three are good answers
3 more 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.
Vacuum leak under hood pass. side near firewall.
maybe your blend door is broken