Not sure what model you have;
The DC models give the following;
F4 bypass contacter tips closed, shorted DTR1, DTR2 or PTR1 (transistors drive left, drive right or pump), bad P9 connector on logic unit.
Driver board defective.
(if its the dc model, check all contacters, one may be stuck).

If its the AC model (starting from the pro 5 series)
Left drive inverter overcurrent.
Failed logic board or inverter.

If its the ac model, check the rs232 connectors on de inverters and check the big connector on the logic board.