GMC Repair: 1998 GMC Sierra Heater, gmc sierra, vacuum leak

Question
It will blow hot and cold and the fan speed knob works but it wont switch modes. (Floor to vent to defrost etc.)Whats wrong? Thanks!  

Answer
If ALL the air is going to the defroster vents especially when accelerating or climbing a steep grade, This is a classic sign of a vacuum leak from the engine source into the interior A/C-heater control unit.


Locate the vacuum line, Usually a thin black plastic line that goes thru the firewall. Look carefully as it may be inside of the wiring harness of which also goes thru the firewall.



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