Did you create a short while adding the new thermistor? Shorts are main reason for seeing def message. Would not really explain why also bed shows it and why they start with correct values. So might also be an other problem.
You can use M105 X0 to also see raw values (at least in 0.92 and dev version). Read analog values are in range 0-4095 and correct sensors show values with a good distance from extremes.
