When a 3-way lighting system fails to turn off even when you toggle the switches, it indicates a specific wiring error or an internal mechanical failure within one of the switches. The most common cause is that the incoming live power wire was mistakenly connected to a brass traveler terminal instead of the dark common terminal, bypassing the switching mechanism and keeping the circuit continuously energized. Alternatively, internal contacts inside an aging switch may have welded together due to arcing. You can troubleshoot lighting faults safely by shutting off the circuit breaker. You can verify that your corrective action is successful when removing the wall plates, correcting misplaced wires so power connects exclusively to the common terminal, and testing the toggles ensures the light turns completely off.
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