I have not checked the ground wire on my 29M. Each time I got that error it was due to the slide not being fully retracted, even if only a fraction of an inch. Just enough to not satisfy whatever measure it uses (motor rotations?). Usually happens after driving, must just creep out a minute amount. If I hold retract, hear the motor spin a few revolutions, hold the button for an additional 5 seconds and error clears.
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