2001 Other GMC Models 6 cyl Four Wheel Drive Automatic 147980 miles
the air does not work and the heat feels like it is on at all times have to drive with the windows down when is is hot out even though the heat is off you can actually feel the heat coming out of the vents everything else seems to be in working order oil is changed on a reg basis
It sounds like the water control valve is stuck, or not working. I think that model is vacuum controlled. Find the valve, ( follow the heater hoses) and check for vac. At the hose on the valve. Either vac. Closes the valve, or no vac.. But you should be able to see the valve move. If all else fails, disconnect the heater lines, and hook them together.