I'm posting this in hopes someone else will learn from my trials.
I have a home designed and built Prusa i3 derivative using aluminum extrusions for rigidity. As such, it has dual Z motors.
I had been trying to get the auto dual Z leveling working and just kept getting errors or each time I tried, the bed would get more out of level. I disabled it for a while in frustration.
Tonight, I went back to look at it again. Same results. The bed would just get more unlevel every time. I was just sitting staring at it and it finally hit me what was wrong. My dual Z motors are run off separate drivers (Drivers 2 and 4), not ganged together in the dual Z plug (Drivers 2 and B). When I built it, I paid no attention to which motor was in 2 and which was in 4. Many of you will just say "DUH!" but, again, I'm hoping this helps someone out. I had them reversed between 2 and 4. The software was working but sending the correction to the opposite motor, therefore making it worse. Face palm. I reversed them and all is well now and auto Z leveling works great.