2001 Chevy Silverado heating and air

Air Conditioning problem
2001 Chevy Silverado V8 Automatic 176800 miles

when I use the heat in the cool mornings, and then try to switch from heat to air, it dosen't want to make the change, and I continue to get heat, even with the air on and the thermostat turned to the left. I was told by a GM factory mech, it could be an actuator valve, if this is true, could you tell me if he is correct and where it is located