I have apartment Norfolk Southern SD40 soon as I program my address into it and like to test it out soon as I went to speed step on it was like on speed step 20 what went wrong and how do I fix
Usually the first thing to do when a decoder seems wonky is to do a reset. CV8, enter 8 and recycle track power. After a few seconds the lights should flash sixteen times for a successful reset and the loco run on address 3.
See if that clears the speed issue.
Rich
Read this for some ideas.
https://tonystrains.com/news/why-decoders-run-away-or-reset/ (https://tonystrains.com/news/why-decoders-run-away-or-reset/)
Sid