About two days ago, I started seeing that my hotend was heating up erratically. I first noticed this while printing a part in PETG and the temp jumped to 260 °C. I shut down the printer at that time and first started checking the hardware. I noticed that the E3D V6 thermistor had been tightened too much. I disassembled the entire hotend, cleaned everything and then reassembled everything. I thought to retune the hotend and when I tried tuning it at 240 °C.
This is where the strange behavior occurs. The hotend steadily climbed up till about 200 °C. After that it just went nuts. I started seeing unreal temps such as 646 °C and such. At this point I thought the MEGA might be at fault. I replaced it and the hotend (an E3D V6 clone). This had the screw on glass thermistor. Again the same erratic behavior and unreal temp readings.
What could be wrong here? What am I missing? Can this be the heater cartridge?