Why Does My 3-Way Switch Only Work When One Switch Is On? How to Fix It

To safely wire a light switch or troubleshoot issues, it’s important to shut off the power at the breaker before attempting any work. Go to your circuit breaker box and shut off the power to the circuit you’ll be working on. 

Once you’ve returned to the switch, unscrew and remove the cover and use a non-contact voltage meter to verify that the power is off. Touch the probe of your meter to the black screw on the switch while touching the other probe to the ground screw. Check that the voltage reading is zero. Repeat this step for each wire connected to the brass screws.

Once you've done this for all the wires and they all read zero volts, it's safe to touch them and begin troubleshooting.

Before you remove any wires from the switch, it’s a good idea to label all your wires according to their location. Use pieces of electrical or masking tape to color code or write labels on each type of wire. 

Don’t forget to snap a quick picture of the wiring set-up to reference when reassembling the switch. If you end up needing to replace the switch, then you’ll be wiring them exactly as they were. If you discover an issue with the wiring rather than the switch, then you can check your photo to avoid wiring it the same way again.

The first thing you should do when troubleshooting a three-way switch that only works when one switch is on is to check the connections. Remove the light switch from the wall by unscrewing the face plate, storing the screws in a safe spot, and gently pulling it away from the wall.

Examine the wiring at the switch that is working properly. This switch should have three wires connected to it, including a black or red wire that is connected to the common terminal, and two other wires that are connected to the traveler terminals. Make sure that all of these connections are tight and secure by tugging on them gently.

Next, move on to the switch that is not working correctly. This switch should also have three wires connected to it. Check all of these connections to ensure they are also tight and secure.

If you spot any loose or disconnected wires, reconnect them and make sure that they are secured tightly. This step might fix the problem, but if the switch still isn’t working properly, you’ll need to check on the wiring setup or the switch itself.

When checking the wiring, you’ll notice four terminal screws: Two are brass-colored, one is black, and the other is green. To ensure proper wiring, confirm that the black and red wires come from the same cable, alongside a white and bare copper wire. These are the travelers that should be connected to the brass screws. They should be identical on both switches. 

Next, you’ll see a black wire coming from a separate cable with only a white and bare copper wire. This wire should be connected to the black screw, known as the common. To ground the wires, connect the bare copper grounds together and land them on the green screw.

Once you’ve completed troubleshooting your switch’s wiring, tie your neutral white wires together with a wire nut, if needed, and carefully tuck them into the back of the box. Fold your wires into the box neatly before reinstalling the light switch and its faceplate, 

Then, turn the circuit back on at the breaker to test whether the issue is fixed. If not, then you’ll need to move onto the last-resort step: replacing the switches.

The last thing you need is to get left in the dark again after you’ve already dedicated the time and energy to fixing the faulty switch. If one switch is bad, it’s likely that the other will also go bad soon enough. With that in mind, it’s always a good idea to invest in the cost of replacing both light switches rather than only the faulty one. If you need to change out multiple switches, including a three-way switch, it’s best to hire a professional electrician to complete this type of task due to its complicated and time-consuming nature.