Same problem with my 2018 GMC Canyon SLE. AC was coming on in "manual" mode, seemingly randomly, with AC off, AUTO off, defog off; even set at ambient temperature. AC came on even when ambient was 40, 50, or 60 degrees and temp set in the 70s. The only way to turn off the AC was to turn of the fan switch which also shuts down the ventilation system. After several visits to the dealer and finally two contacts with GM (First contact recommended turning off auto defog which did not help the problem) Second contact with GM got the following response:
"Keep in mind that these automatic systems will keep the AC compressor on but at a reduced displacement when not needed as much and this helps to minimize moisture buildup on the windows and windshields.
Also, even if the customer does not have the AUTO selected, the temperature regulation is still in AUTO mode unless the temp is set to full hot or full cold. So, if the customer has the temp set to 70 degrees, and the cabin is at 70 degrees, the HVAC system will cut back the temp and could actually feel colder than needed, but it is working to maintain the requested/commanded temp. If the sun is shining inside the cabin and the inside temp increases above 70 degrees, the system will turn down even further and provide cooler/colder air to get back down to the requested temp."
In summary, setting the temp control to full cold or full hot overrides the AUTO function. At full cold, the display on the temp control display reads "LO". (Haven't checked the high side yet but assume it reads "HI") So far, this seems to be working on my vehicle. None of this is outlined in the owners manual.
In reality, there is no "manual" mode as you are either getting air at ambient temp or full hot. There is apparently no in between temp which does not involve the AC compressor. My non-GM vehicles with auto climate control do offer a true manual mode in which you can adjust the temperature without involving the AC. A far superior system as far as I am concerned...
