"84152501 is what my 19 came with. Can’t imagine they changed them for 21 but I have no way of knowing.
Just don’t spend that much. $85 eBay for a set of four. Still more than the $50 option but a lot less than that dealer price."
That is what I was thinking and I also saw the ones that you are referring to and will order those.
"Will the truck recognize more than 4 extra sensors? At different times? I would like to install these on 3 trailers. Two are 2-axle trailers and a single axle trailer."
I believe that you set up a profile for each trailer, but I am TOTALLY new to the new gadgets on my truck. I went from 2008 Duramax to what I have now.
Thanks to all who replied!