I have been having the same problem with my 2016 Ram 1500 big horn. The fan either doesn't come on at start up or shuts off randomly while driving. Sometimes it comes back on and sometimes it doesn't, within minutes or not at all. No rhyme or reason, thought I had a handle on it with it being hot inside the cab, parking in the Florida sun and all. But that is not consistent either, happens when I park in the shade to. Thought it was auto start also but that's not the case because it occurs with a key start also. Fourth time in the shop for me now. Unless it's of when I get there it there is nothing they can do.