The driver's LED serves to indicate a fault condition. The driver chip has built in protection for overheating and over current. If the LED is on, most likely, the driver will stop until the condition is resolved. If you see the LED on, adjust the vref trimmer. First, connect a multimeter to the pads just next to the trimmer potentiometer and adjust until the voltage is lowered a bit. Alternatively, even though the PCB contains a vast amount of thermal vias and a relatively large ground plane connected to the belly of the driver, there still may be a need to cool the driver chip using a small heat sink. Do this only if you are not satisfied with the performance of the motor at the torque require.
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Under what conditions should the LED on the 3d printer driver be lit when the driver is connected to the MB?