MK8 extruder showing 500+ °C on temperature sensor? I just finish building a Graber i3 printer from mixed parts (a MDF clone of Prusa i3 MK2). For the hotend, I've bought the Greetech MK8 extruder. However, the temperature sensor is driving me up the wall.

I've already checked the wires and connections, but the darn thing keeps showing a steady 500 degrees Celsius. In Marlin, it shows it right up (I'm using the 1st option for a 100k thermistor), in Repetier Firmware (using the same one), it shows 0.00°C, until I tell the printer to heat up, when it shoots to 500°C just like before with Marlin. The documentation for this extruder only lists it as a "100K NTC Thermistor", so I tried to select one of the NTC options on the list for both Marlin and Repetier and the temp sensor reads steady 3.600°C!!!

Measuring it with the multimeter, it shows around 60k, it's 31°C outside.

Is it broken or am I selecting the wrong thermistor type?

Documentation for MK8
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