This is case specific, hopefully someone who doesn't already know this will look for help and find this someday. What I had was a 2002 Dodge Ram 1500, 4D, 4wd, AT and all the HVAC controls seemed to function just fine, and you could hear the blower changing speeds, and the AC compressor worked, and yet no matter what position you set the mode control....no air flow anywhere. No air flow from vents; is a good search string.

Anyway...NO, the blower cage wasn't stripped on the shaft, (if someone is reading for fun and guessing that).

On this particular design, if the Recirculation Door, just plain breaks, it can fall off, and down, and sit perfectly flat, BLOCKING the "intake" of the blower motor. So just like if you put a piece of cardboard on the back of a box fan....if a fan can't suck, it can't blow.

The quick rig-fix for the typical no-money customer, was to just pull the broken door out through the hole that the blower motor fits into.

Geoff from Hawaii