Geeetech

Thank you
That exactly what I needed to know.

My problem is that I don't know what type of thermistor is actually in the hotend, so I dont know what the data are to enter ?
If you could provide the identity / type of thermistor mounted, I can easily enter the values.

Yes.
But Repetier host also "see's" the incorrect temperature and Marlin acts according to it.
That is, If I set a extruder temp of 240 C, the actual temp being used is around 260-275 C.

The odd thing is that the heated bed reports the correct temp....

I have a Prusa I3 Pro B printer.

If I measure the temperature of the hotend with a separate thermometer.
Actually a multimeter with a temp sensor.
The external measurement consistently report a 30 - 40 degrees higher temp
than what the Marlin firmware reports.

What temperature sensor type is ...