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Why aren’t the lights on my pre-lit tree working and how can I fix them?

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Q: I have a pre-lit artificial Christmas tree, and it seems like every year fewer and fewer lights work. The tree is only about five years old. I don’t want to buy a new tree. Is there a way to solve this problem?

A: Yes, you can probably fix burnt out bulbs on a pre-lit tree if the bulbs are traditional mini incandescents. There’s also a good chance you can fix a tree with LED lights, as long as they’re not powered by batteries or via a controller.

When mini incandescent lamps were introduced they were usually wired in series, meaning the current needed to flow through each bulb to complete the circuit. If one bulb failed, the entire chain would shut down, as the failed bulb left a vacuum in the circuit. People could spend hours removing each bulb and inserting a new one to identify the problem.

But starting around 1970, manufacturers began to incorporate a shunt function into mini string lights, said John DeCosmo, president of Ulta-Lit Tree Co. in Glenview, Ill. (888-858-2548,, which began selling pre-lit Christmas trees in 1996. When one bulb failed, electricity could continue to flow, leading to marketing messages promising that when one bulb burned out, the others continued to burn.

Still, it’s worth replacing burnt out bulbs when you notice them, as one bulb’s lack of power increases the amount flowing through the remaining bulbs. Four burned-out bulbs in a 50-light set reduce the life of the set by more than 60 percent, DeCosmo said.

Top tips for buying, organizing and decorating with Christmas lights

But what do you do when a whole section of lights stays off? About half the time, a broken shunt is the problem, DeCosmo said. That’s where a tool sold by his company, the LightKeeper Pro ($28.99 at Ace Hardware), can help. Instead of trying a new bulb in each socket until you find where it fixes the problem, you can repair the shunt with just a few clicks of the tool trigger.

To use the tool, plug in the lights and remove a bulb in the area that remains dark. Connect the tool to this socket through a port on the tool, as if inserting a new light bulb. Then pull the trigger on the tool up to 30 times, until the rope re-ignites. Each pull of the trigger sends an electrical pulse through the wires, eventually driving a miniature solder that connects the broken connection. Remove the tool and reinsert the bulb you removed. Unless that bulb has the broken link, it will turn back on, along with all but the one that had the broken shunt. You probably won’t even notice that dim bulb once the others light up.

If an entire section remains dark, there is likely a break in the circuit too large to heal with a mini solder. Maybe a bulb is missing or not properly seated. A voltage detector or the built-in detector of the LightKeeper tool can help you quickly determine where this type of problem is. Plug in the tree and start testing the bulb closest to the socket. If you’re using the LightKeeper, hold down a button on the top of the tool and touch its end to the bulb. If you hear a continuous beep, it means power is getting to that point on the string light. You can then check each bulb or go forward 10 bulbs until you get no more beeps, then go back to find the problem. Add a bulb if one is missing, or remove and reinsert the one that is there. If it remains dark, replace it.

You may have read that a blown fuse could be the reason bulbs stop working. While it’s true that you can replace the fuse (it’s usually built into the back of the socket on a string light), it’s unlikely to explain why the lights on a pre-lit tree won’t work. not, DeCosmo said, so he doesn’t. recommend replacing the fuse first. A fuse blows when a circuit is overloaded. Tree makers are usually very careful to keep the number of lights below what the fuse can withstand, usually three amps, so when a fuse blows it’s usually because someone has plugged in other lights in a pre-lit tree or tied too many string lights together to decorate another space.

What if you have burnt out bulbs on a tree pre-lit with LED bulbs? LEDs are much less fragile and have a longer lifespan than incandescent bulbs, so these trees are less likely to have problems. Even long lasting LEDs will eventually stop working.

Another Ulta-Lit tool, the LED Keeper ($31.99 at Ace Hardware), can help you fix them. To use, plug in the tree and mark where the lights go out. Then unplug the tree from the outlet and plug the tree lights into the tool instead. A YouTube video called “NEW LED Keeper Instructional Video” shows how to test sections of the circuit until you locate the problematic LED. You can then replace the bad LED if you have a matching one. If not, or if the LEDs are not replaceable, the video shows how to replace the bad LED with one of the two “pods” included with the tool. (If you need more pods, a pack of four is $14.99 on Amazon.)

Overall, the LED Keeper has about four stars on Amazon. Dissatisfied customers complain that the threads in their cords are too thin for the tool to work. DeCosmo said skinny wiring is found on low-voltage sets that run on batteries or through a controller to create special effects. The tool cannot help with these string lights or with string lights, nor will it work with incandescent string lights.

A problem at home? Send your questions to [email protected]. Put “How To” in the subject line, tell us where you live, and try to include a photo.

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