Outdoor thermometer reads -40 so the climate control system blows heat in the summer

there could be a air lock in the cooling system and the heater core is full of air
bleed the air out or have an accredited radiator shop do it
make sure that the heater control valve is getting the signal ( cable, vacuum etc ) from the heater control switch ( climate control module) to turn the coolant system on to the heater core

GM products have been having problems with their electrical controlled heat/cooling system..the ativator to the duct doors goes bad..one on the driver side is an easier fix but the other near the center can be tricky and labor intensive to repair..Have a mechanic check it out...but before doing that go the your local auto parts and buy a auto heat thermometer that is designed to read air temp out of the heat ducts inside the car. Then let the engine warm up to operating temp...now turn the heat to full heat and place the thermometer in the main vents on the dash. The temp should read in the 70-80 degree range and your heat gauge should read about 1/3 to 1/2 way on the dail....If low heat open the hood and carefully check the upper hose to the radiator that comes out of the top of the engine through the thermostat..it should be hot to the touch and very firm..if warm and squishy you may have low fluid or sticking thermostat..This time of year when heat is required more often thermostats can stick in the open position not allowing the fluid to heat to the higher temp needed for your heating operation....This is a fairly easy and inexpensive repair and can be a DIY ..hopefully this is the problem and not the door activator....just remember to check the fluid level while the engine us off and cool....be careful and good luck..

not blowing air or when switch is on blows cold air when you have climate control knob turned over to heat? it could be a switch or blower motor or fuse if its not blowing any air. if its blows cold air when knob is on heat then is can be thermostat or low on coolant or it could be climate control module or it could be a air filter in dash.

It sounds like the system is fried. Based on the description, it is just pulling in the outside air and not doing anything to heat or cool the flow. Unfortunately, this is a job for your preferred mechanic.

I would say either blend door issues or a vacuum control valve, heater control valve or a vacuum hose to the heater system is leaking. I would start to suggest. You are correct in checking the blend door. A computer scan should tell you what temp the climate control thinks it is in the passenger compartment. It may be a thermostat problem giving the climate control a false reading. Your blend door is controlled by a servo motor under the dash. You can remove the servo and watch to see if it moves when you change the temp setting from heat to cool. Also, you should be able to manually move the door to the heat position and see if you get heat.

You need to have the temperature valve actuator replaced. It's a dealer only part on top of a huge labor fee (about 600). I've been just opening my windows in the winter when it gets too hot... but with spring/summer coming I will have to have it rigged to constantly blowing cool air (and bring a sweater!). Good luck.

if you are sure that your heater core is not restricted. then try to recalibrate the climate control panel. if your a/c system is not working then DO NOT ATTEMPT THE RECALIBRATION it will make your lights on climate control panel flash codes forever.
to reclaibrate the climate control panel just have the engine running. turn the mode to vent, the temp to cold, the blower on high, then press and hold the rear wiper wash and wiper buttons at the same time until the panel lights up then wait about 2 minutes for it to run completly when completed. turn off ignition and then restart the engine try to se if you have heat again. if it doesntwork post the codes it gives you after the calibration test.

Did you check your coolant level or was it low and you added to it at some point? If so the system my need to be bleed or worst case the thermostat is going bad.

The hot water from your engine cooling system is not getting into the heater core. Check out the valve that admits water into it.

Did you make sure all the air was bleed from the cooling system? to do this jack the car up in the front with a floor jack as high as poss, then fill the coolant recovery tank up, run the engine until it is hot but not overheated, shut the engine down and let it cool, this should purge the air from the system, if the system checks ok then but later starts to act up again the the problem is a leaking head gasket. One last note make sure the recovery tank is refilled, it will **** down.