like another person mentioned look at the sensor in the headliner, it's right above your head when are sitting in the drivers seat, look up it looks like a little microphone/speaker grill with holes in it. That is a ambient temperature sensor, if it is reading the wrong temperature in the cabin it makes the hvac change temperature, they are about $25 and easy to change out, just pull down the headliner from the side, right next to it. It may not throw a code but can still be bad.this is good to know, i need to read my tahoe and see if i have any codes with it. i am getting an odd acting heat. figuring blend doors, but not sure. it will work normal for about 20-30 min then blow cold/ambient air ( not a/c) but if i crank the temp to max it will go back to hot. will be grabbing my htp and taking a look as it will be easy to do.