Nozzle Temperature Cable Failure at EINSY Cable Plug End
After several weeks of reliable operation, my Mk3 suddenly developed under temp failures during prints. The temperature sense cable was intermittently becoming disconnected. That was easy to diagnose because nozzle temp would drop to 0 on the display.
I found the culprit and its root cause. Prusa adds a yellow and green shrink wrap to indicate which cable is the nozzle temperature. This shrink wrap is relatively stiff and long. On my unit, the shrink wrap was applied practically at the connector plug. Very little free wire was between the connector plug and shrink wrap. Because of its close position and length, closing the EINSY cover tilt the stiff shrink wrap and it acted as a lever against the plug housing. It gradually pulled one of the wires completely out of its crimp.
Fortunately, I have the tools to repair the pulled out crimp pin, but it is worth checking that your nozzle temperature lead shrink wrap isn't too close to the plug housing. If it is, I would advise trimming the shrink wrap to leave some free wire between the shrink wrap and plug body.