MK8 extruder showing 500+ °C on temperature sensor?
A brand new thermistor shows about 95-105k ohm (depends on the multimeter quality), so it looks like that one is out of the range.
To be sure that it is a thermistor, you could connect a 100k potentiometer, play with it and see readings on the lcd. If the readings are OK, then mainboard is good so replace the thermistor.

A 100k thermistor curve