I wanted to chime in because I had the Error 18 on my Ninebot ES4 after trying MPC firmware settings that were too agressive and burnt out my scooter going up a steep hill (running from a coyote actually, but that's another story). I found all the discussion about soldering on circuitry on here to be too complicated, so I just bought a new wheel/motor at first. That didn't change anything, so I bought a new control board unit for about $50 and after that did the trick to eliminate the Error 18.
So my 2 cents is that if you have an Error 18 on your ES4, try changing the control board first. Cheap $50 fix on ebay. It's a pain to install, but I managed it after watching a YouTube video. The extra-long needle nose pliers recommended in that video also helped, and it's a good tool to have around anyway.
And now I have a spare wheel/motor :)
FWIW, I had my MPC set as low as it would go because I live on a steep hill and need the climbing power. I'm trying 49000 now and might go lower if that doesn't do the trick. Especially since I'm now an expert at swapping fried control boards. Hahaha