This could be for either of two reasons. If the thermostat is being used in Fahrenheit scale, the Up and Down buttons may have accidentally been pressed at the same time, which changes the display to Celsius mode. To correct this, press and release the Up and Down buttons simultaneously again. Another reason could be that the temperature sensor has been bent. Pull off the faceplate and check the thermistor to see if it is damaged or bent over. The thermistor is a little device sticking up in the bottom right of the circuit board, it will be either blue or black. Make sure it is not damaged and is standing up straight off the circuit board.