Testing and Replacing a Three-Way Light Switch

Three-way switches fail quietly. A light stops working from one location but still responds from another, or flickers in a way that makes you wonder if the house is haunted. The problem is almost always a worn contact inside one of the paired switches, not the wiring between them. What makes three-way switches tricky is not the replacement itself but understanding which wire goes where—these switches have three brass terminals instead of two, and getting them backwards leaves you with a light that works opposite of how it should, or not at all. The testing process takes ten minutes and requires a multimeter and a screwdriver. The replacement takes another twenty once you understand what you're looking at. The key is identifying the common terminal before you touch anything, usually marked with a darker screw or the word 'common' stamped in the metal mounting strap. Map your wires to their terminals, take a photo if that helps, and the actual swap is straightforward. Done correctly, both switches will control the light exactly as they did before, just without the intermittent failure.

Confirm Power Is Off. Flip the breaker for the circuit feeding these switches, then test both switch locations with a non-contact voltage tester against the switch box and any visible wire ends. Three-way circuits sometimes share a breaker with other rooms, so verify the light doesn't turn on from either location before opening anything.
Expose All Three Terminals. Unscrew the cover plate, then remove the two screws holding the switch to the box. Pull the switch out gently—you need to see all three terminal screws and which colored wires connect to them. Note that three-way switches have two brass terminals on one side and one darker terminal (black or copper) on the other, plus a green ground screw.
Mark the Common Wire. The darker screw terminal is the common—this wire carries power in or sends it out to the light. Put a piece of tape on this wire before you disconnect it. The other two terminals are travelers that run between the two three-way switches, and these are interchangeable with each other but never with the common.
Verify Switch Continuity. Set your multimeter to continuity mode. Touch one probe to the common terminal and the other to one of the brass traveler terminals. Flip the toggle—you should hear a beep with the toggle in one position, silence in the other. Move the probe to the second traveler terminal and flip again—now it should beep in the opposite position. If you get no beeps or beeps in both positions on the same terminal, the switch is bad.
Free All Wires. Loosen all three terminal screws and pull the wires free. Match your new three-way switch to the old one—find the common terminal marked on the new switch body or strap. Some switches mark it with 'COM' or a black screw; some have it on a different side than your old switch, which is fine as long as you connect the common wire to the common terminal.
Connect All Terminals. Connect the marked common wire to the common terminal first. Attach the two traveler wires to the two remaining brass terminals—these are interchangeable, so either traveler can go to either brass screw. Connect the bare copper ground wire to the green ground screw. Tighten each screw until the wire can't rotate under the terminal.
Seat Switch Flush. Push the wired switch back into the box, folding the wires in an accordion pattern behind it. Align the mounting holes and drive the top screw first, then the bottom, checking that the switch sits plumb in the box. If wires bunch up and prevent the switch from seating flush, pull it out and rearrange them.
Verify Both Locations Work. Replace the cover plate finger-tight, then restore power at the breaker. Test the light from both switch locations in all four combinations—both up, both down, one up and one down. The light should turn on and off from either location regardless of the other switch's position. If it works backwards or only from one location, the common wire is on the wrong terminal.