Ninebot Max Error Code 21
"Battery Communication Error, Please check the Battery, Control Board & related connections"
This error is thrown when the ESC is unable to receive any data from the BMS/Battery.
This may be because the 4 pin BMS data line has come unplugged or is not firmly plugged into the ESC.
It can also happen when the battery is fully dead or something has killed the BMS board inside.
A failed BMS flash can also cause this error code.
The first thing to check is that the battery had not somehow become unplugged.
Remove the bottom panel and the control board so that you can access the wires more easily.
Unplug and then plug the 4 pin data line back in, making sure it clicks into place in the JST connector.
Plug the scooter into the charger and leave it for a few hours.
If the error is still being thrown, check that the blue LED light is still blinking in the window on the battery.
If the led is not blinking, but the scooter is still able to remain powered on (even with the error code), the BMS may be dead.
If you have a multimeter, it would be wise to check that the battery voltage is somewhere between 30-42v.
If it is, the BMS board needs to be replaced.
Alternatively, the entire battery can be replaced as the BMS replacement is a bit involved.
There are still some unknowns about this error code and as of yet