Home heating and cooling thermostats fail for a number of reasons. One of the big reasons they stop working is due to excessive dust, cobweb and nicotine buildup that can inhibit both mechanical and electrical function.
Another reason that a thermostat may fail to work is there could be loose or corroded wires inside the unit. A thermostat that is not level on the wall may also not function effectively. Thermostats placed in the wrong area of the house will fail to work properly because direct sunlight and drafty windows can interfere with the unit's ability to regulate temperature correctly.