# Two switches, ONE light. How do I know which switch is bad? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [January 12, 2023, 6:49pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124 "2023-01-12T18:49:50Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [January 12, 2023, 6:49pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/1 "2023-01-12T18:49:50Z")
In my bathroom, I have two switches at opposite ends of the bathroom that go to the main overhead light. The problem is, the switches have to be in a certain order for them to work. One switch has to be down in order for the other switch to work. If both switches are up, neither one of them will work until you go turn down the other switch. Is there a away to tell which switch is faulty? Or am I going to have to replace them both? ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [January 12, 2023, 6:58pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/2 "2023-01-12T18:58:48Z")
I’m not sure it would be a faulty switch. Did this just start? if so, faulty switch. If it’s always been doing that, the switches may have been wired wrong in the first place. ---
**Author:** ![Projammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/projammer/32/559_2.png) [@Projammer](https://boards.straightdope.com/u/Projammer)\ **Post date:** [January 12, 2023, 7:59pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/3 "2023-01-12T19:59:13Z")
You’ll need a voltmeter to work it out, but I’ll second what **Sam\_Stone** suggested. It’s more likely that they weren’t wired correctly to begin with unless this is a recent development. ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [January 12, 2023, 8:01pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/4 "2023-01-12T20:01:13Z")
Thanks guys. The switches have worked fine for the 10+ years I have lived in the house. This is a new development. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [January 12, 2023, 8:01pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/5 "2023-01-12T20:01:46Z")
Yeah, sounds like the travelers are incorrectly connected; a common fault even among electricians. You could try switching the travelers on one of the switches and see if that cures the problem. Oops, just saw your post above. ---
**Author:** ![Projammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/projammer/32/559_2.png) [@Projammer](https://boards.straightdope.com/u/Projammer)\ **Post date:** [January 12, 2023, 8:02pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/6 "2023-01-12T20:02:41Z")
Voltmeter then. Can be had dirt cheap at Harbor Freight, Walmart, Auto Zone, Amazon. ETA: Or you can replace both switches (recommended) for less than the price of a meter. ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [January 12, 2023, 8:07pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/7 "2023-01-12T20:07:47Z")
I already have a volt meter. I’m not sure if that will work tho’ bc they are both still getting a charge. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [January 12, 2023, 8:09pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/8 "2023-01-12T20:09:39Z")
> [@Grrr](#): > > The problem is, the switches have to be in a certain order for them to work. One switch has to be down in order for the other switch to work. If both switches are up, neither one of them will work until you go turn down the other switch. Every dual-switch light I’ve had has worked this way. I suppose you could wire one “backwards” so that both up means on but then what happens if both are down? I’m guessing that means on too. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [January 12, 2023, 8:09pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/9 "2023-01-12T20:09:48Z")
If your meter has a continuity function, or resistance function, you can check that the switch is connecting what’s supposed to be connected in the up position. Or, you can spend under $8 to get two new 3 way switches and just replace them. ---
**Author:** ![Projammer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/projammer/32/559_2.png) [@Projammer](https://boards.straightdope.com/u/Projammer)\ **Post date:** [January 12, 2023, 8:09pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/10 "2023-01-12T20:09:48Z")
Then check the terminals on the power side to see if flipping the switch changes the hot wire. If it does, then the other switch is faulty. ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [January 12, 2023, 8:11pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/11 "2023-01-12T20:11:59Z")
Ah ok, I gotcha’. Thanks. ---
**Author:** ![DavidNRockies](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidnrockies/32/6279_2.png) [@DavidNRockies](https://boards.straightdope.com/u/DavidNRockies)\ **Post date:** [January 12, 2023, 8:15pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/12 "2023-01-12T20:15:26Z")
I’m in the “both are right” camp. Properly wired, a set of three-way switches should be agnostic to what the other switch is doing – fully up or fully down. Either switch should be able to turn the appliance on or off. AND you may have blown a switch, improperly wired though it likely was: > **[How to Troubleshoot 3-way Switches - Electrical 101](https://www.electrical101.com/troubleshoot-3wayswitches.html#)** > > If you have problems with a 3-way switch, check out the 3-way switch troubleshooting page. ETA: I’m a bit unclear: is the “both have to be up or down” thing the new part? Of was that always the case, and now one switch no longer turns the appliance on OR off? ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [January 12, 2023, 8:23pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/13 "2023-01-12T20:23:25Z")
> [@DavidNRockies](#): > > ETA: I’m a bit unclear: is the “both have to be up or down” thing the new part? Of was that always the case, and now one switch no longer turns the appliance on OR off? Basically, I have two switches. I can make either one of them work. But in order for the one to work that I want, the OTHER switch has to be in the down position. So if I come in and turn the first switch on, and then I try to turn the light off with the other switch, it won’t work bc both switches are in the up position. Whichever switch is down, the other switch will work normally. ---
**Author:** ![DavidNRockies](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidnrockies/32/6279_2.png) [@DavidNRockies](https://boards.straightdope.com/u/DavidNRockies)\ **Post date:** [January 12, 2023, 8:25pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/14 "2023-01-12T20:25:52Z")
I should just wait for E\_C\_G to post, but … Yeah. In that case, put me in the “buy two new three-way switches, replace both, and be sure they’re wired properly” camp. Removing and testing each of the two is about half-way to replacing them with new hardware 😉 ETA: less than six bucks each: > **[Hubbell CS320W Commercial Specification Switch, 3 Way, 20 amp 120/277V, White:...](https://www.amazon.com/Hubbell-CS320W-Commercial-Specification-Switch/dp/B00FL6YZ7W)** > > Hubbell CS320W Commercial Specification Switch, 3 Way, 20 amp 120/277V, White: Amazon.com: Tools & Home Improvement ---
**Author:** ![DavidNRockies](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidnrockies/32/6279_2.png) [@DavidNRockies](https://boards.straightdope.com/u/DavidNRockies)\ **Post date:** [January 12, 2023, 8:46pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/15 "2023-01-12T20:46:34Z")
Just to add… From that link I posted: > [@](#): > > 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. > > 1. Toggle each switch until the light come on. Toggle one switch to see if it turns the light on and off. If it does, it is a good switch. Leave the light on. > 2. Toggle the other switch to see if it turns the light on and off. If it does, it is a good switch and the other is bad. > > One of these switches will not turn the light on and off (while the light is on) and that is the bad switch. Which is the right troubleshooting approach. But as a general policy, if I have to replace one, I’m going to replace both. If they’re likely the same age and one’s gone bad, figure the other one is circling the proverbial drain. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [January 14, 2023, 9:56am UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/16 "2023-01-14T09:56:49Z")
Tape one of them down. Now you have a room with one functioning switch. Problem solved. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [January 14, 2023, 1:28pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/17 "2023-01-14T13:28:13Z")
If I didn’t want to diagnose them, I would buy one, as you need one, replace one and if everything works I would just do the happy dance. If it does not fix it I would take that old one I just pulled out and use it to replace the second one, then go grab a beer, job is done. Actually I would diagnose it, open them up and test them and look for loose wires, listen and feel the action of the switched, also note which switch is used more often, but either way all I say all you need is one switch. I would start with suspecting the one that is only working in the single position, which is acting like a standard switch (not 3-way). > [@DavidNRockies](#): > > But as a general policy, if I have to replace one, I’m going to replace both. If they’re likely the same age and one’s gone bad, figure the other one is circling the proverbial drain. Switches can last many many years and can go at any times, and in the case of 3-way’s one is typically used far more then the other. I wouldn’t jump into that boat unless it’s for for aesthetic reasons. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [January 14, 2023, 2:58pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/18 "2023-01-14T14:58:10Z")
> [@kanicbird](#): > > If I didn’t want to diagnose them, I would buy one, as you need one, replace one and if everything works I would just do the happy dance. > … This. The whole post, I just truncated it for brevity. Pick the most used switch. Replace it with a new one. If that doesn’t work, take the switch you removed and replace the other switch with it. Less work than trying to troubleshoot it. Every time I have messed with three-way switches, I get dumber. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 14, 2023, 3:15pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/19 "2023-01-14T15:15:48Z")
> [@Alessan](#): > > Tape one of them down. Now you have a room with one functioning switch. Problem solved. So when switch A is down, switch B is able to control the light? And when switch A is up, the light is off no matter what? If so, then I suspect an open contact in switch A: [![Imgur](https://live.staticflickr.com/65535/52626956348_e62b455216_b.jpg "Slide1") ](https://www.flickr.com/photos/194366829@N05/52626956348/in/dateposted/) [![Imgur](https://live.staticflickr.com/65535/52625966162_31ba68fcfb_b.jpg "Slide2") ](https://www.flickr.com/photos/194366829@N05/52625966162/in/dateposted/) ---
**Author:** ![Novelty\_Bobble](https://avatars.discourse-cdn.com/v4/letter/n/a3d4f5/32.png) [@Novelty\_Bobble](https://boards.straightdope.com/u/Novelty_Bobble)\ **Post date:** [January 14, 2023, 3:58pm UTC](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124/20 "2023-01-14T15:58:06Z")
ETA - never mind, I see my point was already made [Next page](https://boards.straightdope.com/t/two-switches-one-light-how-do-i-know-which-switch-is-bad/978124.md?page=2)