How to fix a three-wire Christmas light string

Yesterday we had a three-wire Christmas light string where the first half of the string was working, but the second half of the light string wasn’t working. We couldn’t fix it by replacing the bulb right where the light string stopped working, so I looked into it and found this helpful (but a little too technical) article.

The solution

The short version of that article is that the way these light strands work is that the light bulbs are connected in series and in parallel, and the most important thing to know is that the bulbs seem to be connected in pairs. For us, the “pair” part was a key, and through a little trial and error, we found out that the bulb where the light strand stopped working had indeed burned out, but it was also joined up with a second bulb further down the line that had also gone out. Once we replaced both bulbs, the entire light strand lit up.

I didn’t have any special tools for this, so what we did was just start at the first bulb that wasn’t working, and then we tested every other bulb in the strand by removing it and testing it with another strand. We found several bulbs that were in one of three states:

  • The bulb worked
  • The bulb didn’t work, but the rest of the light strand worked
  • The bulb didn’t work, and it caused the rest of the light strand to fail

I replaced about four bulbs until I replaced one light bulb and the entire strand came to life. I assume this bulb was paired with the first bulb in the strand that was out, but I have no way of knowing that for sure. The keys for me were (a) knowing that the bulbs could be in one of those three states, and (b) just taking the time to test and replace all the bad bulbs until the strand started working again. It seemed like a lot of work initially, but I found the problem bulbs in less than twenty minutes.

If you have a problem with a set of three-wire Christmas lights that aren’t working, I hope this solution is helpful. I don’t own any special tools, so the solution isn’t very elegant, but by going through the light strand and replacing all of the bad bulbs, I eventually hit on the one (or two) bulbs that were causing the whole strand to fail.

Good luck, and Merry Christmas. :)

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