This just recently happened to me on my WW 1.0. It ended up being the drive motor on the left side. To trouble shoot. I jacked up gantry and put bricks under. Then ran the motors. With the bad side not engauged it would not trip it would work. See my previous post for jacking up gantry with a Harbor Freight ram. Much easier than any other way and also makes any motor or gear box work much easier.