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I've found a lot of answers to this question after replacing a light switch but this has happened without any work done to the light.

I have a living room light that is connected to a standard light switch on the wall. No dimmers or anything fancy. Seems like a standard two-wire scenario in the box. Last night when my wife flicked the switch on one of the two bulbs went out. Not terribly unusual for a bulb to go out. When it came time to go to bed though, she found that the switch didn't turn the light off.

I flipped the breaker off and the light went off but as soon as I flipped the breaker back on, the light turned back on as well. I can replace the switch since it looks like it is very old. My only thought is maybe the switching mechanism is no longer stopping the flow of electricity but the fact that the bulb also burned out at the same time has me concerned that somehow we have a short in the walls or something.

Is this as simple as hooking up a new light switch or does it sound like there's more going on?

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    @Harper-ReinstateMonica you're presuming a cheap LED, which the OP didn't state at all. Incandescents burn out somewhat regularly, and some people actually still buy then. On purpose! – FreeMan Dec 29 '20 at 20:06
  • It was an incandescent bulb, not LED. I tested the switch with the multimeter and I'm getting a circuit connected from the posts even when the switch is "off" so I think it's a pretty open and shut case with the underlying issue. The bulb burning out is hopefully just a coincidence (though an odd one). – Zombian Dec 29 '20 at 20:58
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    If there were never coincidences we wouldn't have a word for it. – Mark Ransom Dec 30 '20 at 4:12
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A "short" as you call it seems unlikely since that should cause the breaker to trip.

As a next step, you should SHUT OFF THE BREAKER, and then check the switch since you already seem to believe that it may be bad. Yes, switches do go bad and so your belief is reasonable at this point. If you have a VOM (Volt-Ohm Meter) checking the switch is simple enough. If not, you really should get one but another approach is to simply change the switch (new ones are inexpensive) and see if that resolves the problem.

If that turns out to be the problem, then you should be able to replace the switch and move on.

Otherwise, more troubleshooting is in order.

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    Thanks! I have a "healthy" respect for electricity (aka I'm scared to death of it), so I didn't even think about testing the switch with my multimeter. Tested the switch and sure enough I'm getting Ohms beeps when in the on or off position (tested for false negatives on the adjecent switch which only beeps when on, as expected). I will buy a new switch but the multimeter test tells the story pretty definitively. I'm going to say the bulb burning out was a coincidence. – Zombian Dec 29 '20 at 20:54
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    I've had some cheaper bulbs fail pretty violently, sometimes violently enough to trip a breaker. It wouldn't surprise me if said surge could also cause an already marginal switch to weld shut. – Peter Green Dec 30 '20 at 1:31
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    @Zombian a healthy respect is good. Electricity can screw up your day and burn down your house. Take your time, double check your work, and check your assumptions before committing. When in doubt, stop and reevaluate. Also stop when your test results don't match your expected results. You can do this, just take your time. – Criggie Dec 30 '20 at 8:07
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    @PeterGreen, I worked side-by-side once with a guy who claimed to be one of the inventors of the arc-fault circuit interrupter. I never verified his story, but... He claimed that the biggest challenge was making it not trip every time an incandescent bulb blows. He said that the blue flash that you sometimes see and the "pop" that you sometimes hear is an arc that can carry up to 300A for a half-cycle of the AC waveform. – Solomon Slow Dec 30 '20 at 19:32
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    @zombian If the light had burned out at some other time, then I would guess that they were unrelated. But since it appears that it burned out just when the switch was turned on, and that was likely the same time that the the switch shorted, I'm guessing they are related somehow. The weak switch may have fused which led to a surge which caused the light to fail. Or vice-versa. Either way, you just replace the switch and it's all good. – BWhite Dec 31 '20 at 4:43
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The contacts in switches can definitely "weld" or stick in the closed position or the switch can just wear out so it doesn't separate the contacts. Turn off the power and examine the switch, just replace it anyway and see what happens. The bulb burning out could just be a fluke.

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Your initial response is correct that the switch is faulty and should be replaced. When you mentioned that it is an old switch this gives more credence to the claim to replace it.

The one issue to check is that the bulbs are not CFL's (fluorescent) or that the bulb is set tight in the base. If not, either might cause intermittent problems. Change the CFL bulb if there is one.

Changing a 2 way switch is a simple matter of putting the wires on the terminals (after the power is shut-off). Post a photo if you need more help.

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    Yes a CFL can be kind of flaky, but there's no way it will light up if the switch isn't allowing it to get power. But you're right, changing a 2-way switch is about the simplest electrical DIY project you can get. – Mark Ransom Dec 30 '20 at 4:16
  • Assuming it is indeed a 2-way switch. There might be another switch somewhere - one house I lived in we discovered one of the lights had a second switch which had been stuffed inside the wall and plastered over by a previous occupant. People do all kinds of shady stuff like that. – Darrel Hoffman Dec 30 '20 at 19:10
  • @DarrelHoffman, I'm still mystified as to why the switch for my living room ceiling fan is in the bathroom. – Matthew Dec 31 '20 at 16:12

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