Hi Jim. I think the thermistor needs to be calibrated. I imagine there must be something in the software to do this. You could also test to see if it has gone bad. If you know the type of thermistor you can get the datasheet for it and run some tests on it (in boiling water, or air gun) and see if the ohms match the datasheet.
Also if the thermistor for the hot plate and the extrude head are different, they may have been swapped during build. The hot plate thermistor may only be good for lower temperatures with poor accuracy at higher temperatures.