Thermal runaway on newly assembled prusa i3 mk3  

Thermal runaway on newly assembled prusa i3 mk3

Hi,

I've just received a prusa i3 mk3 kit and assembled it. It's my first 3d printer. After successfully printing 3-4 excellent prints with very good quality, my printer seems to randomly lose power to the hotend and the temperature drops like a ball when it does. This result in the error: "Thermal Runaway". The heatbed works fine. The hotend temperature sensor works as usual with correct readings. I've tried to reinsert the power cable to the hotend, but that does not do the trick.

Performing a selftest after a software reset fails when checking the hotend with the error: "Hotend: thermal/not connected". I'm quite certain I've narrowed down the problem to the hotend power cable, but I don't know hot to proceed to find the error.

The cable seems to work at random, If I just try to preheat the printer, it sometimes works. Then I can print as usual. But a thermal runaway is expected within a few hours.

How should I proceed? What is most likely to be the error? I have a multimeter but I'm not really certain how to use it to narrow down the problem further. I would really like to solve it without tear down the extruder head, but seems necessary at this point.

I could also mention that I've hooked up a raspberry pi to it. The problem appeared after I installed the raspberry pi, but disconnecting it does not seem to solve the issue, so I don't think it's related to that. I was probably just lucky to not have the error before installing the raspberry pi.