If you are seeing a “def” error message next to the extruder and/or bed temperature indicators on your LCD screen then you most likely have a thermistor problem. The thermistor is what allows the extruder and also the heated build plate to operate at a certain temperature.
Why are you getting the “def” error?: You have a bad/cut/melted/frayed/disconnected thermistor and/or thermistor cable. To find the problem spot you will need to inspect the thermistor and the thermistor cable carefully for anything that resembles a problem. The usual problem spot is at the point of contact with the heating block. This is due to the amount of heat that is generated from the heating block and also because there is pressure on the thermistor cable by the screw and nut set. Also be sure to check for a loose cable connector at the mainboard (micro-controller) as these are known to un-plug out of place.
The Solution: Once a thermistor is malfunctioning it should be replaced. However, if the problem is a cut or disconnection somewhere along the cable it can be repaired/re-connected. If you have never repaired a wire it is crucial that you ask us for help by contacting us at firstname.lastname@example.org since this wire is what will regulate how much heat is transmitted to your extruder assembly (possibly very dangerous). If you have connected wire before then the procedure is standard: create 2 clean cut ends, pull back cable sheathing exposing the metal wire ends, take one of the ends and slide on heatshrink tube all the way past the point that will be soldered, solder both new wire ends to each other, slide heatshrink back over the solder point, shrink heatshrink, and lastly wrap in kapton tape for extra safety.