Step-by-Step

A low-voltage system begins with a transformer that is either mounted to a panel on the furnace or connected to an electrical box. Anywhere from two to six thin wires (depending on how many items are being controlled) lead to the thermostat base, where they are connected to terminals. The thermostat body contains the heat-sensing device and the control dial. Because the voltage is so low, it is not necessary to shut off power to the thermostat while working on it--unless you are working on the transformer.

Dust can cause a thermostat to malfunction. Remove the cover plate and brush the inner workings with an artist's brush. Pay special attention to dust and dirt on contacts.

Unscrew the screws that hold the thermostat body to the base, and pull the body away. Check to see that the base is securely fastened to the wall. If it is loose, the thermostat could tilt, which would throw off the settings. Blow on the body to remove more dust, but do not handle the parts inside -- they are sensitive.

Look for loose, corroded, or broken wires coming into the base. If any are damaged, clip them, strip insulation from the ends, and reattach. Tighten all the terminal screws to make sure the connections are secure.

Cut a short piece of wire, and strip insulation from both ends. Use it to "jump" between terminals. Touch one end to the "R" terminal and one to "W," and the heater's burner should come on. Touch "Y" and "G," and the fan should come on. If they do, the thermostat is faulty and should be replaced.

If the thermostat checks out, test the transformer. Touch one probe of a voltmeter or multitester to each of the low-voltage terminals on the transformer. Set dial to ACV 50. If the meter does not detect current, the transformer is defective and needs to be replaced.

Before you go out and buy a new transformer, open up the transformer box and make sure that there is power leading to the transformer. Touch one probe of a tester to the hot wires and the other to the box (if grounded) or the neutral wires.