Denali n DOO said:
I was just wondering if you had solved this issue? I too am having similar problems, since yesterday I believe. I have the digital dual automatic climate control. Yesterday I had the windshied replaced, probably not related but I saw Roadie mentioned that twilight dome thing on the dash. So when I got it back I noticed the external temp read 16 when in fact it was about 25-27 outside. I did the combined button push, mode+front def+rear def. It then started at 41 and kept counting down and stopped at 27. Perfect, so I thought. But now I get same as you did. I start at 15 and cool air comes out, I increase one digit at a time and the air temp stays the same all the way to 31, as soon as I hit 32 the heat comes blasting out. Not sure if I screwed it up by pressing those 3 buttons together. I wonder if I should pull that HVAC #36 fuse for a reset or may this cause more problems? let me know what your outcome was if you could. Anyone else have a suggestion?
The Auto HVAC system has a few more sensors as Roadie explained. It also prefers if you set and forget it, because it can respond somewhat slowly to control panel changes.
I have now and have had GM's Auto HVAC systems in many vehicles including my Envoy and the sensor most overlooked is the INSIDE Ambient sensor. This sensor draws interior air across a thermistor and informs the control panel. The control panel usually buffers this signal and combines it with outside air temp to decide what to do.
In the Trailblazer/Envoy/variants line, the interior sensor is to the left of the drivers head in the B pillar (with a small round grille) This sensor contains a tiny fan (not to be confused with the Blower fan) to draw the interior air across the thermistor for accuracy. A few things can cause inaccurate readings. Most likely is dust accumulation, also the tiny fan motor can slow and/or stop from "inhaling" dirt and dust.
When this tiny fan motor and/or the thermistor become dirty, it ends up giving readings closer to outside temps. (outside sheet metal is not far away). This usually causes too much Hot air on cold days and too much Cold air on hot days. I am not aware of a code for the failure of the tiny fan motor.
Occasionally I blow the dust and dirt out of the sensor while installed, but have had times on other vehicles (2000 Blazer LT) where I had to remove the sensor and clean it while checking for proper tiny fan operation.
FYI:
Sometimes I point my leftmost Driver dash vent up and over towards (not directly at it) the sensor to "help" it get an airflow.
In response to the Dash sensors, there is a Left and a Right Sunload sensor on the Automatic HVAC (one for each zone) and the Ambient Light sensor for the Auto Lights shares the same central Dash housing, but is not part of the HVAC system. Sunload sensors can go bad and should set a code.