Bad wiring, DTR1 defect, Current limit set to low, Defective DD2/DD2, defective driver board, defective current sensor or failed logics.
My expierence is that F5 is usually a defective transistor due to a bad drive motor> higher resistance of the motor and power cables (or bad carbon brushes, damaged collectors) lead to excessive currents wich could lead to faulty transistor (DTR1).
Also wise to check the setting of the current sensor, they can be adjusted with a small internal potmeter.
Lots of options...
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