Three-way switches are used to control an individual light or an electrical outlet from two separate places in a room. When you turn on the switch and the light doesn't come on, you should first check to make sure the light bulb didn't blow out. Take out the light bulb and put it in a lamp. If the light bulb works when you turn the lamp on, you should next check the switches to make sure they haven't gotten worn out. This is something a normal homeowner can do with a continuity and circuit tester. Here is how you can test and replace a worn out three-way switch.
Turn Off Power
You should turn the power off to the switches at the main circuit panel, but you still need to test each switch to make absolutely sure the power is off. Use a screwdriver to remove the screws for the cover plate and then remove it. Take out the screws holding the switch to the electrical box, and remove the switch.
You will see three screws along the side of the switch. Each screw will have an insulated wire connected to it. One screw is dark wires attached to screws on the switch. This screw is called the "common terminal screw," and the hot wire that supplies power is supposed to be connected to this screw. However, this doesn't guarantee that whoever put in the switch did it correctly.
You need a circuit tester to check to make sure the power is completely off. A circuit tester has two probes; one is colored black and the other red. The circuit tester also has a little light in it that will turn on when it detects power. Place one of the probes against the metal electrical box the switch is in, or the copper grounding wire connected to the bottom of the switch, and place the other probe on each screw one at a time. If the light never goes off, you know you don't have power going to the switch.
You want to mark the wire going to the common terminal screw before you remove any wires. Place a small piece of masking tape around the wire. Unscrew and remove all the wires from the switch; including the grounding wire.
You need a continuity tester to make sure the power flows through the switch like it should. A continuity test has a clip on it. Place the clip on the common screw, and then place the tip off the continuity tester against the other screws one at a time while flipping the lever on the switch up and down. If the light on the continuity tester doesn't come on, or if it comes on when the lever is up or down, then the switch is bad. If it only comes on when the lever is in one position, then the switch is good.
Replacing the Switch
Once you get a new switch, put the common wire that you have marked on the common screw, and then attach the other two wires to the other screws. These wires are interchangeable and both can go on either of the other screws. Connect the copper wire to the grounding screw. Put the switch back in the electrical box and replace the cover plate. Turn on the power. The light should turn on now when you flip the lever on the switch.
For more help, contact an electrician.