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OP you may wanna check the DIY sticky up top although as already mentioned the blend door is stuck or you are low on freon...my bet is the door. Need to replace the actuator
Are you now or in the recent past had battery trouble?
If so it's more then likely the blend door, I just went through the same issue on another GM car with auto air.
The battery went dead and I replaced the battery and the AC blew hot air on the drivers side only after the replacement.
I pulled the fuse to reset it and it did nothing, then I removed the positive battery cable for ~5 minutes and it works again!
thanks for the quick response, sorry if I posted to the wrong section, removing power seems to have fixed it. if it happens again I will look at actuator