the speed sensor can be checked on the display but not always accurate. it can be checked with a meter that can read frequency. connect to negative and test the brown wire and the black wire with a white stripe (backprobe) and run machine. they should read relatively evenly. if one chanel is failing the controller can not sense direction of rotation immediately and may cause the problem you are having. if you need instructions on using on board diagnostics let me know and i will help you through it but for this test the on board is not very good. if i may i would like to ask you something, are the two motor controllers interchangeable on this unit. they are on ours and it is a good diagnostic tool.