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Summarize the problem

After replacing several light switches on the same circuit (one switch at a time), one light fixture will no longer light up. The light fixture is controlled by two three-way switches. The light fixture's hot wire is getting 120v (tested with a multimeter). I've tried four different three-way switches and two different light fixtures. I checked the twisted neutral and it is secure. The rest of the light switches on the circuit work perfectly (bathroom, hallway, and front door lights).

Circuit includes:

  1. Upstairs light (will not light up, the remaining items work perfectly)
  2. Bathroom light and fan
  3. Hallway light, exterior light, and front door lights
  4. Doorbell

Provide details and any research

There are two three-way switches at the top of the stairs. The switch on the left controls a different circuit that works perfectly. The switch on the right (referred to as Three-way switch #1) controls the light fixture that won't light up. Three-way switch #2 is at the bottom of the stairs, on the same circuit as switch #1 and the problematic light fixture.

enter image description here Three-way switch #1 (right), breaker on, red traveler wire is cold (three-way switch on the left can be ignored, it works perfectly)

enter image description here Three-way switch #1, breaker on, white traveler wire is cold

enter image description here Three-way switch #1, breaker on, black wire is hot

enter image description here Three-way switch #1 installed (right) - ignore the switch on the left

enter image description here Three-way switch #1 installed (right) - ignore the switch on the left

enter image description here Three-way switch #1 wires

enter image description here Three-way switch #1 twisted neutral wires

enter image description here Three-way switch #2, white traveler wire is hot

enter image description here Three-way switch #2, red traveler wire is cold

enter image description here Three-way switch #2, black common wire is cold

enter image description here Three-way switch #2 wires

enter image description here Three-way switch #2 installed

enter image description here Light fixture black wire is hot (120v, tested with multimeter)

enter image description here Light fixture white wire is cold

enter image description here Light fixture installed, will not light up (also tried another fixture with the same result)

enter image description here Fixture black and white fixture wires: 0v

enter image description here Fixture black and ground wires: 120v

enter image description here Switch #1 black and white (neutral) wires: 0v

enter image description here Switch #1 black and ground wires: 123v

enter image description here Fixture connected white (neutral) and ground: 0v

Other switches on the circuit

enter image description here Bathroom light switch (left) and fan switch (right)

enter image description here Bathroom box

enter image description here Bathroom neutrals (retwisted after)

enter image description here Hallway light switches (hallway light, two front door lights, and exterior light on the side of the house)

enter image description here Hallway box

enter image description here Hallway neutrals (retwisted after)

enter image description here Interior doorbell

enter image description here Exterior doorbell (faceplate removed)

enter image description here Circuit breaker (#34) that powers the problematic upstairs light fixture, as well as the bathroom light and fan, hallway light, exterior light, two front door lights, and doorbell

When appropriate, describe what you’ve tried

I've tried:

  1. Four different three-way switches
  2. Two different light fixtures
  3. Testing the hot wires with a multimeter to confirm they're outputting 120v
  4. Untwisted and retwisted the neutral wires at switch #1
  5. Trying multiple combinations at switch #2 (black wire on black screw, travelers on brass screws; white wire on black screw, travelers on brass screws; red wire on black screw, travelers on brass screws)

I think the problem is that I have an open neutral on another light switch. Is it possible that an open neutral on one light switch could break the circuit on another light switch? Anything else I can do to get this light fixture working?

Thank you.

Solution

I finally fixed this, and boy do I feel dumb. I did not realize that, in addition to all the light switches, there's an outlet on this circuit. I had replaced the old dual outlet with an LED light and single outlet, and it was not working. I took the wall plate off, removed the outlet and found the problem. For some reason, I only connected one neutral to the left screw and capped off the other neutral. That was the open neutral. I connected the capped off neutral to the screw, in addition to the other neutral, and now the outlet and light fixture works!

enter image description here The open neutral

enter image description here Closed the neutral and now the light fixture works!

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    This has got to be one of the most thoroughly written, clear, well-documented questions I've ever seen on any of the stacks I've visited! Well done! – FreeMan Jun 9 '20 at 15:53
  • Glad you were able to find it! Post that Solution as an answer and I'll give you a +1 for it :) – ThreePhaseEel Jun 9 '20 at 22:41
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The pictures are great. To my eye it does appear you've wired the 3-way switches correctly; I think you're right that neutral is missing. You can confirm by measuring voltage (with the multimeter) between black and white at the light with the power on. Measure voltage from black to ground there as well. Do the same check at the switch #1 box: measure voltage hot-neutral and hot-ground. For extra points, connect the light fixture and then measure voltage between neutral and ground.

It seems likely you'll find ~120 V hot-ground at both locations and some floating voltage hot-neutral. If you have the light connected you'd probably also see ~120 V neutral-ground. All of these conditions point toward neutral being open somewhere along the way. You're probably going to have to chase this upstream of switch #1.

Content of other boxes

I count four cables in the bathroom switch box. It looks like the pair in the middle are the switched lines to the light and fan; the outer cables must be the supply coming in and going out.

The hallway switch box also appears to contain four cables. Since there are 3 switches and no /3 cables, this box must be the end of the line for the circuit -- there isn't a cable carrying hot and neutral on to anywhere else. The neutral bundle looks a little suspect - the stripped part of one conductor goes down much lower than the others do, which makes me think it may not have been pushed up into the wire nut as far as it should have been. It probably just slipped down while you had the wire nut removed, and anyway this does seem to be end-of-line.

Focus on getting neutral working at the switch #1 location first. It's probably easier to reach than the light is, and you also have an always-hot wire there. By measuring the unswitched hot to neutral you avoid the risk of accidentally collecting false data (ie measuring at the light while the switched hot is turned off).

Could there be any outlets in this circuit? Verify neutral continuity through those too.

Tracing conductors

You may have come to the point of tracing out conductors. There are many ways one could do this, most of them laborious, and with various levels of risk of shock or equipment damage.

  1. Measure resistance. Do this with a de-energized circuit. Disconnect both ends of a conductor and measure the resistance of the conductor. Do the same with other conductor(s) in the same cable. This helps to confirm that a cable goes where you think it goes, and that all its conductors have continuity. If the meter leads don't reach you can extend with some wire, an extension cord, alligator clips, etc. Could also measure loop resistance, for example: short black to white at one end and measure resistance between them at the other end, then try shorting black to ground and do the same.
  2. Use a cable tracer. In the world of telephone and Ethernet there's a tone generator and inductive pickup kit often used to trace wires. This can be used for tracing mains wires too. Even though the tone generator can withstand the 90 volt POTS ring voltage, this probably should be done with the circuit de-energized.
  3. Use a test lamp. This is done with the circuit live; appropriate precautions must be taken. One would open up the connections in the switch boxes, identify which cable brings the hot (and neutral) and start adding back one cable at a time, using the test lamp to look for hot and neutral showing up at other boxes.
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  • Thank you! I posted pictures and captions of the five scenarios you outlined. Fixture black and white fixture wires: 0v. Fixture black and ground wires: 120v. Switch #1 black and white (neutral) wires: 0v. Switch #1 black and ground wires: 123v. Fixture connected white (neutral) and ground: 0v. Do you think there's an open neutral upstream of switch #1? – Ryan Jun 8 '20 at 22:56
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    Yes, since the hot-neutral voltage at switch #1 is 0 and hot-ground is not, your open neutral must be upstream of that point. Of course it's also possible you have a more severe/perverse wiring fault yet to be found, but let's keep going with the open neutral theory a while longer. – Greg Hill Jun 9 '20 at 0:00
  • I just updated the post with pictures and captions of the other switches on the circuit (bathroom and hallway) under the "Other switches on the circuit" header. I untwisted and retwisted the neutrals in both boxes, but I'm still getting 0v from the light fixture hot to neutral wires. What else do you recommend? Thank you! – Ryan Jun 9 '20 at 15:56
  • Greg - thank you so much. There is an outlet on this circuit that I wasn't aware of! I updated my post with photos and more detail under the "Solution" header. I had miswired this outlet when I replaced the old outlet and the new outlet wasn't working because of an open neutral. I correctly wired it and now the outlet and, more importantly, the light fixture works! Thank you so much for your help. I'm so happy to have this finally fixed. Thanks again. – Ryan Jun 9 '20 at 22:14

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