I ran into this problem with a set of blend motors myself. It was quite a b*tch to get that to work. I have 3 pairs of motors and 2 hevac units and between the lot I got it to work out. I first connected a pair of know good motors to the hevac unit and send them to the middle position. I switched these to the other hevac unit to clear the error (and keep it cleared) then I started messing around with the other motors on the spare hevac to get them to behave Once I had the set I wanted to use in a state where they were NOT on their ends (some I had to manually move with a power supply) I cleared faults in the spare hevac and positioned the motors in a known good position. I then proceeded to mounting the motors so that they could not accidentally overrun (they have slightly less movement when mounted) and connected them to the hevac unit I intended to use. Once "booted" I could run the calibration test and voila! no errors and the motors have been working since.
I'm pretty sure some steps I have taken are not needed but that is what I did to get it working. ymmv.
Good luck!
-P
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