How to fix 3 wire christmas lights

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Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?

My prelit tree has strings of LED lights and each bulb has three wires going to it. Looking on the web there are a number of sites that describe how to repair strings wired in series that have two wires per bulb. However I cannot find any reference to strings with three wires per bulb.

The joyous holiday season is nearly upon us, but there’s one tradition I hope to avoid this year: dealing with burned-out Christmas lights.

Each holiday season I unpack, untangle, and plug in the previous year’s mini lights, only to discover they don’t work. Sometimes individual lights are burned out, and other times the entire string seems dead. Are these lights worth saving, or should I just toss them out and buy new ones?

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I admit that diagnosing and fixing Christmas lights can be frustrating, but with a little determination and a few specialty tools, you can repair most faulty light strings.

How to Fix Christmas Lights

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1. Diagnose the problem.

Carefully inspect each string of lights before plugging it into an electrical outlet. If you see cracked or slit insulation, frayed or bare wires, or damaged plugs, discard the string.

Now, determine if it’s actually a bad bulb causing the malfunction or something else. If it’s a smaller light set, it’s likely wired in series: the electrical current passes through each individual bulb in order to complete the circuit and illuminate the set.

Larger light sets often have two or more circuits wired in parallel, which explains why sometimes just a section of the string goes dark. In most cases, simply replacing one bad bulb will fix the entire set or one darkened section of it.

2. Find the bad bulb, and swap it out.

Locating the one faulty bulb that’s causing the problem can be tricky. The best option is to use either an electrician’s multimeter or a tool specifically designed for repairing Christmas lights, such as the Lightkeeper Pro. It combines a voltage detector, bulb remover, bulb and fuse testers, and shunt repairer in one compact tool.

You may be able to reveal the bad bulb by simply plugging the lights into an electrical outlet. If you’re lucky, the “bad” bulb isn’t actually bad at all. It may just be loose and needs to be pressed down more firmly into its socket.

Also, remove the bulb and look closely at the two tiny wires protruding from its base. They should be firmly attached and not touching each other. Plus, each wire should be laid flat against the outside of the bulb. When you push the bulb back into its socket, the wires complete the electrical connection.

You can also test the bulb using the LightKeeper Pro, or a multimeter. If you discover the bulb is burned out or damaged, replace it with a new bulb. Just be sure it has the proper voltage rating or you risk damaging the whole light set.

3. Fix the faulty filament or shunt.

If all or part of a string of lights is dark, the problem may be a broken filament or a faulty shunt. (A shunt is a device that allows current to continue flowing through a circuit by creating a path of lower resistance than the original path. In incandescent holiday lights, shunts are small wires wrapped beneath the filament.)

If a bulb’s filament breaks, the shunt redirects current through the base of the bulb, maintaining the electrical circuit. A faulty shunt may sound like a catastrophic failure, but you can often fix it with the LightKeeper Pro.

Simply plug in the light strand and remove a bulb that’s in or near the dark section. Next, insert the tool into the bulb’s socket and squeeze the trigger to activate a piezo circuit. A high-energy pulse will shoot through the set and after about 20 pulses any faulty shunt should be re-activated. Pretty cool, right?

If you’re still having trouble locating the broken circuit, try using a voltage detector. You can buy one for about $10 or so, but there’s also one built into the LightKeeper Pro.

4. Replace the fried fuse.

If the voltage detector doesn’t find anything wrong, there’s one more thing to try: Check the tiny fuse located behind a small sliding door on the male end of the plug. If the fuse is fried (you can check it with the LightKeeper Pro), replace it with a new one of the same amperage, which should be indicated on the plug. If after all the testing the light string still doesn’t illuminate, it’s time to get some new lights.

5. Upgrade your lights.

If you’re buying new lights, consider upgrading to LED Christmas lights. There are many benefits to LEDs that make them a great choice over incandescents. They’re more energy efficient, last much longer, and can be programmed to display different colors and patterns. Plus, LED lights are more durable and less likely to break than traditional bulbs. Here are some of our favorite string Christmas lights for indoors and out.

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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.

We 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 after it 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. :)