How can you tell if a three-way switch is bad?

Determining whether a 3-way light switch has failed internally and requires replacement involves performing continuity testing with a digital multimeter. First, always ensure safety by shutting off the circuit breaker and verifying zero energy with a voltage tester before removing the switch from the wall box. Second, disconnect all wires from the switch terminals. Third, set your multimeter to the continuity setting, place one test probe on the dark common terminal, and test against each traveler terminal while toggling the switch handle back and forth. You can troubleshoot switch failures safely. You can verify that your test is accurate when the multimeter tone sounds in one toggle position and goes silent in the other, confirming whether the internal mechanical contacts are healthy or broken.