The first thing to do if your thermistor is reading the incorrect temperature is to double-check that you have selected the correct thermistor type in your firmware, for more information about this please visit our firmware documentation page:https://e3d-online.zendesk.com/hc/en-us/sections/360004773037-Lite6-Firmware-Guides- Whilst you are in the firmware settings it is also worth double-checking that you have the correct board type selected as the firmware might be reading the wrong pin for the temperature reading.
If your temperature readings are fluctuating you can do a PID tune for your hotend, there are different methods depending on the printer and firmware you are using so you should find out the correct procedure for your particular setup. If the temperature still fluctuates after a couple of PID tunes at the temperature that you will be regularly printing at then you may have a damaged internal wire that is causing the issue, you can check this by gently moving the wire to see if the temperature jumps.
The second thing to check is the resistance reading of the thermistor, you should do this by removing the thermistor from your hotend, being careful not to pull on the fragile wires, and making sure the thermistor cartridge is not touching anything conductive.
Take a multimeter and set it to the Kohm setting (if you have the option select 200k) Take a reading from the wire connector and write it down. You will then need to take a temperature reading of the ambient room temperature, you should aim to take both readings at the same time, try not to leave it too long between readings as this can ruin the results.
We have a table detailing the expected resistances based on ambient temperatures via the following link:
https://e3d-online.zendesk.com/hc/en-us/articles/360017254038-Thermistor-Table
If you suspect that you have a loose wire or your resistance reading is wrong then you should contact support, or order a replacement thermistor here.