Had the same issue on a 7FBCHU25 truck, I went back to change voltage back to 48v from 36v with no luck. Confused as to why I could change from 48 to 36, but not back to 48 from 36; I went back to the programming section again and again. Finally I found that if I exited the programming at function #6; which happens to be "END" all works just fine. Previously I think I may have just turned off the key or something? to reset out of programming or service functions. Good luck!!
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