I think I've found it. I have two brand new Hexagon hot end kits that came with thermistors, fans, heaters, etc. One of the thermistors had a bad connection to the lead wire, a short. Goes to show, just because it's new doesn't mean it's flawless. One lead had just the smallest bit of solder holding it to the lead wire and it came loose. I've now used crimps instead of solder and I think I'll be OK now.