@rainerunsinn Yes, I found it too. One of the thermistor legs was broken. After soldering it works fine.by pilyugin - Prusa i3 and variants
I have the same issue. I followed this video too. When the printer is just loaded tempetature of hot end is shown as 26 C° and -51 C° for heated bed, but next second both of values are changed to "def". When I plugs printer to computer with Repetier this message is shown in log: "heated bed: temp sensor defect". Also when I try to Preheat PLA (like on this video ) nothing happens. Leds on the bby pilyugin - Prusa i3 and variants