Respectfully, It is complex and not readly fully expainable on a board like this. (no drawings, no test equipment, no specs.!
Service disc is the better option to use as it will be usefull for many issues in the future if you are going to own these trucks over a long period of time.
You know we can't tell you what is wrong over this medium without accurate feed back and test procedure and some test equipment, and it would take quite some time.
Funny, I try to help with education on this board and often get attacked for it, like people 'expect' the right, simple, low cost answer all the time and that's not the way it is.
You own a computer controlled vehichle. I did'nt design it or make it but this is the way it is. Understand it or let a dealer fix it.
Sorry it's not the answer you wanted to hear.
Good luck.