Hi error code E71 is height- low voltage that could be anything from the computer to any wire harness, but if you try to drive and loose power on the display first thing to check is the battery must have 48v or no less then 42volts when is driving put your multimeter on the battery cables and look at the meter what is the voltage! try to press on the accele, very slow look at the meter shouldn't go lower then 42 volts, the error code will stays on until is clear of throught atlet service manual, but the unit should work, what the model and serial will help? My guess is your battey!