<p>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&nbsp;°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&nbsp;°C. </p>

<p>This is where the strange behavior occurs. The hotend steadily climbed up till about 200&nbsp;°C. After that it just went nuts. I started seeing unreal temps such as 646&nbsp;°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. </p>

<p>What could be wrong here? What am I missing? Can this be the heater cartridge? </p>

This can come from several sources:
Hardware
The thermistor or its connections might be damaged, and the fault is only observable when the hotend is hot or moved to a certain area. Start by checking the wiring! You may be able to repair a bad connection easily, but depending what was broken, you may need to replace something. In some cases squishing a thermistor cartridge too much can destroy the internals, so a replacement is needed.
A mainboard failure is more likely to just show a static temperature, and a heater failure would show as maybe not getting past a certain point.
Firmware
If it had not worked before or you changed the firmware, the firmware should also be a suspect. The firmware can 'fail' when using the wrong thermistor type/table which can result in a very big offset or bad slope, resulting in wrong or unaccurate readings.