2006 GMC Sierra • V8 2WD Automatic • 106,579 miles

On my 2006 GMC Sierra the a/c heater mode stays on vent even when changing the control to the floor or defrost positions. Also the highest fan speed does not work, all other speeds are ok.
Bill Reaver
December 18, 2010.

Its your control unit I belive. Same thing happened to one of my friends 2008 chevy tahoe. You should try that.

Dec 18, 2010.
I need you to check a couple things. First, check if there are vacuum leaks under the dash. The air flow vents are actuated by vacuum. If there is a leak, this can happen. Next, as far as the fan speed, I would recommend checking the blower motor resister.

Let me know what you find.

Thank you for your quick reply.
Where would I look for the vacuum lines under the dash? When I look under the dash no vacuum lines are visable. If it was a vacuum leak wouldent the the engine be running rough? There's no roughness that I notice.
Also a few monthe ago I was running the a/c heater blower on high speed when I started noticing a (electrical) burning smell, then it quit. The blower works on all other ranges but just not on the high speed. Could this be the resistor? Where would I find the resistor.

Bill Reaver
Dec 19, 2010.
I have attached schematics for you to review. The GMC model uses electric and not vacuum for some reason. Review the schematics to see if you can identify where to check under the dash. Also, as far as the high speed, it believe it is a relay rather than the resister. See schematic.