There are two possibilities of cable damages:
- connectivity (wire cut in half)
- short circuit (wires of two cables touching each other or the frame)
The quickest way of diagnosing a problem is simply by replacing the components
affected by the errors you encounter. Use of a second smart module is advised, as
nearly all issues can be traced back to the smart module, because it is sending the
signals in the first place.
A multi-meter with continuity function can be used to check for connectivity and short
circuit. For this you remove the smart module and the affected component. And
connect the multi-meter probes in the socket pin and cable connector. Some bike
doctors use an old connector (the motor for example) and solder all the wires
together. This allows to check the socket pins one by one while having the probe on
the same cable the whole test duration. Be aware that this excludes the testing of
short circuits between the wire.