Determine which switch is not working. You do this by turning each switch on at both locations. With the light on, go to each switch and flip switch to opposite position. The switch that turns the light off is the good switch. Replace the other switch as follows...
First shut off the circuit breaker or fuse that controls the light. You can have a second person flip the breakers while you watch the light go out. (If you are controlling an outlet, plug something into the outlet.) If no second person is available to help, then you will have to shut off breakers yourself by looking at your power panel (hopefully it is labeled) and start turning off breakers until the light goes out.
Now remove the switch plate cover and set aside. You will then be looking directly at the switch. See Figure.
After verifying that the circuit is off, use a screw driver and remove the mounting screws at top and bottom of switch. Pull the switch straight out toward you as far as you can. Do not try to pull switch beyond the length of the attached wires.
Remove the wires from the top two screws at the top of the switch. Separate them and then remove the bottom wires and separate. (Push right wire to the right and left wire to the left.) The wire on the bottom left side is the ground wire and should be gray or green in color. The wire on the bottom right screw is the power lead from the electric panel. This wire should be black, but could be red. See second figure.
Open the package of the new 3-way switch. It should look similar to the one pictured above. It should have 4 screw terminations -- 2 at the top of the switch and two at the bottom. The screw on the left will be smaller and lower on the left hand side of the switch. If it only has 3 screws, it is the wrong switch....
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Author: Ron Ambler