Ninebot ESX Error Code 42
"External battery communication error, check external battery, control board, connected cable"
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Error 42 on a Ninebot ESX scooter is related to issues with the external battery's data lines communicating with the ESC.
Often it is the result of the two mounting screws coming loose or the bracket bending due to wear.
Occasionally issues with the battery itself that are unsolved even after appropriate tightening and cleaning is done may cause this issue, but the fix and cause of that are currently unknown.
How to fix Ninebot Error 42:
Remove the external battery, taking care not to damage the screws as occasionally they are rather soft and can be easily damaged.
Check the terminals, make sure that they aren't dark and coated in carbon deposits.
If there is any sign of a carbon deposit on the terminals, give them a shot of contact cleaner. You should be able to see a good amount of metal on the terminals.
Check the bracket to ensure that it is not bent. If it is bent and you are unable to bend it back into shape, a new bracket can be purchased and may alleviate the issue.
After that, make sure there exist at least 2 small washers on the three screws and 2 thicker washers on the two longer screws.
Use a blue semi-permanent Loctite/thread lock to ensure that the screws stay in the next time the scooter is reassembled.
If there is still an error 42 after all these steps, something is likely wrong with the battery.
This can be confirmed by plugging the battery into a different controller and if it also reads error 42, the battery is most likely broken.