How does Prusa3D firmware respond to an open or shorted thermistor during a print?
This sounds like a problem I had with my hotend cooling fan a couple years back. To answer your question, from what I'm reading, the Prusa3D firmware detects thermal runaway if the hotend temperature drops for more than 45 seconds, it detects an open-circuit by reading 16&nbsp;&deg;C or less (MINTEMP error), and detects a short by reading 310&nbsp;&deg;C or higher (MAXTEMP error). 

Couple questions that might help better troubleshoot the root of the problem:


have you measured the thermistor's resistance while cold or hot?
have you made sure your hotend cooling fan is at an appropriate speed while heated?
have you made sure your print cooling fan duct isn't pointed at the hotend block due to being jostled?


You might find that there are some fairly simple solutions to a handful of overheat problems. I bought a cheap knockoff all-metal heatbreak and immediately ran into problems, but solved them by printing a better heatsink shroud and upgrading to a 40&nbsp;mm fan. If you've upgraded to an all-metal hotend, you might run into issues with PLA specifically if you're not printing enough material fast enough, as PLA is somewhat sensitive to heat creep through the filament itself in the throat of the hotend.