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Why don’t the lights on my pre-lit tree work, and how can I fix them?

5 min

Q: I have a pre-lit artificial Christmas tree, and it seems as if each year, fewer and fewer of the lights are working. The tree is only about five years old. I don’t want to buy a new tree. Is there a way to troubleshoot this?

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

When incandescent mini-lights were introduced, they were typically wired in series, which means the current needed to flow through each bulb to complete the circuit. If one bulb failed, the whole string would go dark, because the failed bulb left a gap in the circuit. People could spend hours taking out each bulb and inserting a new one to pinpoint the problem.

But beginning around 1970, manufacturers began building a shunt feature into mini-light strings, 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 a bulb failed, power could continue to flow, leading to marketing messages promising that when one bulb burned out, the rest would keep burning.

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

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

To use the tool, plug in the lights and remove one bulb in the area that stays dark. Connect the tool to that socket via a port on the tool, as if you were inserting a new bulb. Then press the tool’s trigger up to 30 times, until the string relights. Each press of the trigger sends a pulse of electricity through the wires, eventually resulting in a miniature weld that bridges the broken connection. Remove the tool and reinsert the bulb you took out. Unless that bulb had the broken link, it will relight, along with all but the one that had the broken shunt. You probably won’t even notice that one dark bulb once the rest light up.

If a whole section remains dark, there is probably a break in the circuit too wide to cure with a mini-weld. Perhaps a bulb is missing or is not seated properly. A voltage detector or the LightKeeper tool’s built-in detector can help you quickly determine where this type of problem is. Plug in the tree, and begin testing at the bulb closest to the plug. If you’re using the LightKeeper, hold down a button on the top of the tool and touch its tip to the bulb. If you hear a continuous beep, it means power is getting to that point on the light string. You can then check each bulb or move 10 bulbs ahead until you get no beeps, then backtrack to find the problem. Add a bulb if one is missing, or remove and reinsert the one that’s there. If it stays dark, replace it.

Perhaps you’ve read that a blown fuse could be why bulbs stop working. Although it’s true that you can replace the fuse (it’s usually built into the back of the plug on a light string), it’s unlikely to explain why lights on a pre-lit tree aren’t working, DeCosmo said, so he doesn’t recommend replacing the fuse as a first step. A fuse blows when a circuit is overloaded. Tree manufactures are generally very careful to keep the number of lights below what the fuse can handle, usually three amps, so when a fuse blows, it’s typically because someone plugged other lights into a pre-lit tree or linked too many light strings together to decorate another area.

What should you do if you have burned-out bulbs on a pre-lit tree with LED bulbs? LEDs are far less fragile and have a longer life span than incandescent bulbs, so these trees are less likely to have problems. Even long-lasting LEDs will eventually stop working, though.

A different Ulta-Lit tool, the LED Keeper ($31.99 at Ace Hardware), can help you fix these. To use it, plug in the tree and mark where the lights are off. Then unplug the tree from the outlet and instead plug the tree lights into the tool. A YouTube video titled “NEW LED Keeper Instructional Video” shows how to test sections of the circuit until you locate the problem LED. You can then replace the bad LED if you have a matching one. If you don’t, or if the LEDs aren’t 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 package of four is $14.99 on Amazon.)

Overall, the LED Keeper has about a four-star rating on Amazon. Unsatisfied customers complain that the wires on their strings are too skinny for the tool to work. DeCosmo said the skinny wiring is found on low-voltage sets that run on batteries or through a controller to create special effects. The tool can’t help with these light strings or with rope lights, nor will it work on incandescent light strings.

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