I partially had the same problem you did. I had a faulty blend motor, like we all do tend to get eventually, and I replaced a defective one with a spare from a wrecker. Of course, within a few months the problems returned so I replaced them all with new ones. Unfortunatly the potentiometers were out of range a bit and that causes all sorts of havoc on the HEVAC unit. I replaced the HEVAC unit with a spare, replaced the blend motors with 3 different sets I had laying around until finally it worked.
All I can figure is that the HEVAC is picky and has a bit more memory then we would like, even if you clear the error codes. If the HEVAC detects for only a fraction of a second that the potentiometer input is above (I believe) 125% and below 0% it simply shuts down that blend motor, even manual override no longer works. In orde to prevent this from happening you should try and set them by hand with a power source directly on the wires of the motor (opening them up as a last resort) and mounting the motor inside the heater block so as to prevent over running.
Come to think of it, perhaps they named the hevac wrong and it was supposed to be called havoc? :P
Cheers and good luck!
-P
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