How to Troubleshoot and Fix Holiday Christmas Lights

Hands fixing christmas lights that went out

@erich / Twenty20

Whether your holiday light show involves decorating a small indoor tree or setting up an award-winning outdoor display featuring thousands of lights, that cheerfulness is sullied—or at least delayed—if those strings of holiday lights fail to light up.

Before throwing those strings of lights away and buying new ones, take some time to examine and fix them, if possible. Here's an easy troubleshooting process for checking your strings of Christmas lights and repairing them if they're faulty.

Tools and Supplies You Will Need


  1. Make Sure the Circuit Is Carrying Power

    It sounds too simple to be true, but often problems with a string of lights have nothing to do with the lights. When testing lights, start by making sure you're plugging them into an outlet that's carrying power. If the string of lights appears dead, there's always the chance that the circuit's breaker has tripped and isn't providing an electrical current to the outlet.

    Check the main service panel to make sure the circuit breaker is in the ON position, and reset it if it has tripped. Then, use a circuit tester at the outlet to make sure it is carrying power.

  2. Check the Fuse on the Light String

    Modern sets of mini lights have a small cartridge fuse found in the cord plug, inside a small sliding door. Slide this door open, pry out the small glass fuse and examine it closely. Some light strings may have two fuses; examine them both.

    It's usually possible to see a break in the small metal filament inside the glass fuse if it has burned out. If the fuse is bad, replace it with a duplicate of exactly the same size and rating. Electronics stores or hardware stores sell a variety of replacement fuses.


    A fuse can also be checked using a common electrician's tool called an ohm meter. The goal is to test the fuse for continuity—if the fuse is operating correctly, the fuse will show no resistance.

  3. Check the Individual Bulbs

    Often, the problem is with individual light bulbs in the string. This is easy to spot in some light strings because you can see the individual bulbs that have gone dark. Sometimes the light bulbs will simply be loose in their sockets—just pressing them firmly in place sometimes restores the function.

    In older lights strings, though, a single dead bulb may cause the entire string to go dark. It can be quite time-consuming to replace individual bulbs until you find the culprit causing the problem.

    The job is easier with a Christmas light bulb tester—an inexpensive little tool that you can buy online or from many hardware stores. There are different types of testers available, including touchless models that you simply hold close to each individual bulb to identify if it is bad. With other tools, you remove individual bulbs and slide the plug portion into a small socket on the tool to test it.

The Lightkeeper Pro Tool

If none of the individual bulbs are faulty but the light string still refuses to light up, there is a chance the shunt wires in the light string are faulty. When working correctly, there is a wire shunt in the string that bypasses each of the individual light bulb sockets. This is designed so that power continues to flow when an individual bulb is burned out. But if the shunt wire itself is faulty, the light string will fail to operate correctly.

The Lightkeeper Pro is a trigger-operated tool that sends a pulse of current through the light string to identify the faulty light bulb and repair the shunt. The tool comes in a kit form that includes extra light bulbs, a continuity tester, and even a battery tester for testing various batteries.


If you have old strings of incandescent holiday lights, they may generate considerable heat. Because new styles of holiday lights are affordable, it is a good idea to get rid of old incandescent lights that may pose a risk of fire. These lights are especially dangerous if you decorate with a natural evergreen tree or pine boughs that can become tinder dry. If your holiday lights get painfully hot to the touch, you are well-advised to replace them with strings of cool LED lights.