Joining this Blend Door conversation late. Last summer in extreme heat and smoke (forest fires) on trip through Rockies (BC), passenger (lower, by feet) Blend Door quit, and blew HOT air into drivers side, even on AC max cold. Only relief was to turn on defrost to cold, and max cold and fan speed in rear cabin upper vents. YouTube re calibration only worked for short time.
My 05 Tahoe has 4 Blend Door Actuators, all identical: 2 in front dash, 2 in rear cabin.
When got home, replaced this bad Blend Door, then recalibrated - worked til recently (see below).
In Dec, temp dropped below zero. Now rear heat didn’t work - only blew cold air. YouTube again... There are 2 Blend Doors in rear cabin - front one (by rear passenger door) controls air flow (upper vs. lower), other one (mid cabin) controls heat to cool setting. Once at operating temp, played with ceiling console controls, able to determine bad Blend Door was ‘heat to cool’ located mid cabin.
Removed trim, replaced bad one (mid-cabin) with real AC Delco, recalibrated. It WORKED!, and the rear fix also affected air flow throughout, including front (recent clicking noise returned, even though was replaced).
Front cabin replacement from last summer was auto parts store generic, so will be changed again with AC Delco.
TIP: easier to access the passenger side Blend Door (lower) ACCESS which controls drivers side IF remove Center console cup holder... still need to work from passenger foot area, but another hand through the top makes it easier for re and re.
Hope this helps some of you...