Left Motor test fails

im not sure if this is exactly the issue being discussed but i have a left motor issue.

my left motor fails test 1 out of 2 100% of the time. i have replaced

  • the left motor
  • the left motor wire
  • the shield and arduino

at this point i do not understand what it could possibly be. it seems to operate correctly when cutting or when moving the sled around on the board manually but i have never gotten it to pass all the tests.