This Old House host Kevin O'Connor gets pro tips for finding—and replacing—a faulty three-way light switch
1. Start by turning off the power to the circuit you'll be working on at the main electrical panel.
2. Test the switches with a voltage tester to ensure the power is off.
3. Remove the cover plate from the switch box with a slotted screwdriver.
4. Unscrew the three-way switch you suspect isn't operating properly from the box.
5. Before disconnecting the switch wires, label each wire for easy reattachment later, and mark the "hot" white wire with a black marker.
6. Remove the suspect switch from the box and use a continuity tester to test its operation. If you find that current is allowed to pass through the switch, regardless of the position of the on/off toggle, then replace the old switch with a new three-way switch.
7. After tightening the wire connections to the new switch, wrap electrical tape around the switch.
8. Screw the switch to the electrical box, and replace the cover plate.
9. Test the new switch by flipping on and off the three-way switches at each end of the circuit.