Please explain this

Tooluser

I fit new 100K thermistors to my Mendel, hot bed and Extruder, I set the firmware to Sensor 1 for both. Yet when I go to heat the bed and extruder in Host the starting temperature for the bed is 130 degrees when cold and 147 degrees for the Extruder when cold. They both start to heat but temperatures fluctuate up and down with no stability and then one or other cuts out. Changing the sensor type in firmware makes no difference to either. So what else needs to be changed to set the starting temperatures to room temp before heating?? All good advice appreciated, Thanks all.

Repetier

At 25°C they should all show 25°C as that is the reference for 100K. If it doesn't happen you should measure the 4,7K resistor near thermistor pin. Some boards have the wrong resistor mounted and then all the math does not work of course. The system looks normally like this (might be vise versa)

GND-4,7K- sensor pin - thermistor - 5V

So voltage measured at sensor pin depends on resistor.

But still with wrong value sit should not fluctuate. That only happens through crosstalk or bad contact somewhere.