How to Locate a Bad 3-Way Switch

Usually when a 3-way switch fails, lights can be toggled on and off at one switch, but not the other. If one of the two 3-way switches toggles the light(s) on and off, the other 3-way switch has probably failed. Follow these steps to find which switch is bad. Make sure there is power to one of the switches.

Toggle each switch until the lights turn on. If the lights do not turn on, proceed to "How to Test a 3-Way Switch" below. If the lights turn on, toggle one of the switches to see if it turns the lights on and off. If it does, that indicates it is a good switch. Leave the lights on and proceed to step 2.
Toggle the other switch to see if it turns the lights on and off. If it does not, that indicates it is a bad switch. Proceed to "How to Test a 3-Way Switch" below.

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Note: This article only applies to conventional 3-way switch wiring.

Switches are spring loaded to minimize arcing and prolong their life. However, a very small amount of arcing is present when a switch is toggled. A bad switch may not fail completely, burned contacts may cause intermittent failure. Keep this in mind when troubleshooting switches. Intermittent failure could make troubleshooting difficult, especially with 3 and 4-way switches. If one 3-way switch needs to be replaced, the other one should be replaced too.

If the 3-way switches never worked properly, it is possible they are wired incorrectly. If testing the switches indicate they are both working properly, see “Identify 3 and 4-Way Switch Wires”.

How to Troubleshoot Three-Way Switches

How to Test a 3-Way Switch

Turn off power to the switches at the circuit breaker and using a tester, make sure power is off. Pull the two 3-way switches out from their switch boxes so you can access the switch terminals. The wire connections do not have to be removed. Find Multimeters
Using a continuity tester or the continuity setting on a multimeter, check continuity between the common (black) terminal and each of the traveler terminals (brass colored) of each 3-way switch.
There should be continuity between the common terminal and only one of the traveler terminals. After toggling the switch, there should be continuity between the common terminal and only the other traveler terminal.

When a 3-way switch fails, there will usually be no continuity between the common terminal and one (sometimes both) traveler terminals of one of the 3-way switches.

3-Way Toggle Switch Animation