Usually when one side is hot and the other is cold, and each side is controlled separately, the problem is a blend door not working.
The blend doors are usually controlled by small electric motors.
You might research "blend door" online and see if anything pops up.
SOURCE: 2005 GMC Sierra 1500 - heater blows, but only cold air
Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.
Testimonial: "You were absolutely correct. Low coolant level. Put in some 50/50 and we're nice and toasty now, but need to find out why coolant was so low. Thanks."
SOURCE: 2005 GMC Sierra Denali - Installed new battery; driver a/c hot
need to go to dealer to have HVAC module reprogramed. whenever diconnecting battery power on these vehicles you can connect a 9 volt battery between neg and pos to keep the memory in the computers. I am an master ase technician and 5 classes from gm world class tech.
SOURCE: 93 gmc sierra heater not working
THERMOSTAT REPLACE IF NOT SEE IF HOSES ARE CLOGGED SHOT OF AIR COULD HELP OR DISSCONNECT BATTERY CABLE 10-15 MIN CONNECT BACK TRY HEATER
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We kind of narrowed it to the blend door, but how do you get to the blend door?
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