Make sure there is nothing environmental causing the thermostat to read incorrectly. Check for things such as: 1)The unit is in the path of the exhaust of a fan cooled device. 2)The wall that the unit is located on is being indirectly heated by a device on the other side such as a television, lamp, or other heat-generating electrical device. Clean the thermostat: Using the brush attachment on a vacuum cleaner, vacuum the outside edges of the thermostat to remove any dust that may have collected on the temperature sensor. If the problem is not being caused by any of the conditions described above, reset the thermostat. If this does not work, replace the thermostat.