Going through that now with my 855 purchased in April. I've only used it occasionally up to now but have 10 acres to cut down that hasn't been touched in a year so that will be a chore. I cut for a few hours recently with the deck fully raised until the idiot "hot" light came on. I immediately shut it down and let it cool. Upon investigation, I discovered a small crack at the top of the radiator that wasn't there before. Once I dug deeper, I learned why. There is a simple screen you pull out from in front of the radiator - you don't even need any tools. Just lift up the screen from in front of the radiator. That screen was completely plugged so no air was getting through. As a result, it boiled off the coolant and cracked the radiator. I hope that was all that was damaged.
Now learning how impossible it is to source a replacement radiator (without forking out $1300 to JD), I am hoping what I have can be repaired.