Defetcive temperature sensor or your Oven Needs to Be Calibrated
The calibration dial will be located on the back of your temperature knob or down at the bottom of the small shaft. You should be able to see an adjustment screw on the dial. This screw is where you can adjust the temperature. To increase the temperature of the oven, turn the screw clockwise. To decrease the oven temperature, turn the screw counterclockwise. In most ovens, a half of a turn should increase/decrease the oven temperature by 25 degrees.
USN - RETIRED NUKE