Wife's daily driver, 2004 Tahoe blowing barely lukewarm air. Defroster is marginal, heater no better.
Engine up to 205 degrees, hoses both warm-hot.
Already tried pulling HVAC fuses, which did reset all heat controls : Still no HEAT.
How can I determine Heater core , vs Blend door trouble ?
Which blend door should I pull apart first ?
Which door does the air TEMPERATURE ?
The diverter between Defrost and HEAT is functional.


Wife needs HEAT, quick !

 

When you move it from cold to hot, can you hear it?

What is your coolant level?

It is probably the blend door, they go frequently, it should be right above the transmission hump.

 

There are three blend door actuator motors under the dash. One at the very top is for fresh outside air. One beside the steering column directs the air to the floor, dash vents, windshield. The one you need to check is on the passenger side under the dash. You can remove the actuator motor and physically open or close the blend door with the leaver to test for heat/ac.

 

Dicked around most of Saturday:
Removed the actuator on passenger side, took it apart, re-distributed the clean high grade OEM grease among the reduction gear-train, wiped off the rheostat slider track, assembled the actuator unit, pulled HVAC fuses, hooked actuator up to wiring, plugged fuses back in, and watched the actuator go through it's normal KEY ON RE-Learn cycle, looked all good.
Assembled actuator to blend door, and as long as rotary TEMP ***** are set to 90 degrees, we have decent heat, and defrost.
At lower temp settings, the system seems to " Get Lost" . and revert to blowing cool or cold air.
It appears that the trim level specific DIGITAL HVAC controls, with the rotary temperature *****, and DIGITAL temp display (seems to ) get interior cab temperature data from a complicated thermistor/ fan inducer packaged in a small speaker -looking round bezel, in headliner above driver's head.
Without a good Thermistor reading, the HVAC can get confused, so intermediate, "Reasonable" temperatures, say 79 degrees or 82 degrees won't close the blend door and begin CAB HEAT process.

The simple, field expedient override process is to crank both drivers and passenger TEMPerture ***** to the maximum indicated 90 degrees, in each of the heat, Defrost and Vent modes, after which we have effective, sufficient heat, defrost, and everybody is happy.