Mine does this also although my focus is a MK1 whereas yours is the newer focus. Mine is because of a faulty dash though it's a known issue with my year of focus but what I am saying is it could be your dash at fault not just the sender. The sender is the first thing I would change though as if your fans are constantly on then it might be a simpler fix. My gauge doesn't work (or sometimes doesn't work not all the time) due to a known fault with my dash.