I've had a few years experience on these trucks, and I've noticed with these motors mounted in the horizontal position and depending on the environment these trucks are working in, debri and muck can gather around the drive motor terminals and if it's a bit damp can trigger fault codes, removing muck and dirt usual rectifies problem.
You can do a lot of calibration through the dash display on this truck but you do need an engineers no to access the menus and I'm thinking you may need the acceleration pot calibrating.

Nero