When in idle, the temperature on the screen should be almost the same as the ambient temperature. If you see a very high or low value on the display, it might indicate a problem on the thermistor or on the board. The most frequent cause is a shorten thermistor.
Check the connections specially the wires that go to the thermistor located on the hotend. Most of the times is the screw that secures the thermistor that shorts the wires.
Unscrew the small screw that holds the thermistor and check if the temperature changes.
Replace the thermistor.
If the problem persists, then the problem might be in the analog input of the microcontroller on the board.
Replace the board with a new one.