A thermostat senses when the air temperature moves above or below the desired level and switches the heating or cooling system on or off.
If a thermostat is attached to an electrical box and has standard house wires connected to it, it is a line-voltage unit. If it malfunctions, shut off power to the circuit, remove the thermostat, and take it to a dealer for a replacement.
A low-voltage thermostat receives power from a transformer, much like that for a door chime. Typically the transformer is located on or near the heating or cooling unit. Replace the transformer if it does not deliver power. In a standard setup two wires control the heater and two control the air-conditioner. If there are more wires, consult a heating and cooling pro.
Less than 1 hour for most repairs
Screwdriver, soft brush, strippers
Stripping wire, making connections
Short length of thin-gauge wire, perhaps a replacement thermostat or transformer
A thermostat that sends the wrong signal to the heating unit or air-conditioner may just be out of plumb. Use a string with a small weight near the two alignment marks. If they do not line up, loosen the mounting screws and twist the thermostat until the marks align.