Check the batteries. Most times the battery is the culprit. The problem with all those electronic thermo/hygrometers is that the power is supplied by the battery and that power is not regulated. The sensors rely on electricity to function properly and that electrical current must be constant to remain accurate. As the battery runs low on voltages, the power available to the sensors is reduced and the result is inaccurate reading. Also, do a double check if you are using rechangeable batteries. Most of the AA or AAA batteries are 1.5V where as the same battery size in a rechargeable is probably only 1.25V. That 1/4 volt difference will cause your electronic thermometer/hydgrometer to give false or inaccurate readings.