How to Find a Bad Bulb in Christmas Lights

Don’t let a broken strand of lights zap your holiday spirit

couple decorating christmas tree with lights
Photo: JGI / Tom Grill / lend Images / Adobe Stock
couple decorating christmas tree with lights
Photo: JGI / Tom Grill / lend Images / Adobe Stock
Get quotes from up to 3 pros!
Enter a zip below and get matched to top-rated pros near you.
Geo Icon

A faulty light strand can be a major holiday headache, especially if you’ve already spent time hanging it or incorporating it into your festive decor. Luckily, you can learn how to find a bad bulb in Christmas lights before you resort to the tried-and-true (and time-consuming) method of replacing and testing each individual bulb. This guide will walk you through how to find the bad bulb in Christmas lights before calling a professional holiday decorator to solve this issue. 

1. Test the Extension Cord 

If your lights are plugged into an extension cord, start by checking whether the cord is the source of the issue. Unplug the cord and plug the lights directly into the outlet to see if they work.  

2. Check the Extension Cord, Circuit Breaker, and Outlet

In some cases, the issue is the circuit breaker or the outlet rather than the strand of lights itself. 

  • First, if your outlet or extension cord has one, check your circuit breaker to ensure it's set to "ON." 

  • Next, plug in a working device into the outlet to verify that it's functioning properly. 

  • You can also use a continuity tester to test the outlet. 

  • If you believe the outlet could be faulty, it's best to contact a local electrician to come and fix it.

3. Inspect the Wires

To check a strand of Christmas lights for damaged wires, start by unraveling the lights and inspecting them for broken wires and sockets. If you find any wires poking out, it's best to replace the entire strand. 

4. Check the Fuse Cartridge

fuse cartridge on a string of lights
Photo: JJ Gouin / Adobe Stock

If all the lights on your strand aren't working, it's possible that a blown fuse is the culprit. Testing and addressing this problem is relatively simple.

  • Find the male end of your lights. 

  • Then, use a screwdriver to slide open the housing cover and access the fuse. 

  • Check the fuse for signs of burning or a broken filament. 

  • If you notice any damage, replace the fuse. 

  • Once the new fuse is in place, slide the cover closed and plug the lights in to see if they work properly.

5. Check for Loose Bulbs

woman holding blown out string of lights
Photo: Tracy Packer Photography / Moment Open / Getty Images

Another common reason for malfunctioning Christmas lights is loose bulbs. Here’s how to troubleshoot:

  • Unplug the light strand. 

  • Inspect each bulb by gently pushing them into their sockets to ensure they are properly seated. 

  • If you find any bulbs that are not fully seated, replace them with new ones. 

  • Test the lights again.

6. Use a Christmas Tree Light Tester

If you've exhausted the previous solutions and your lights still aren't working, try testing your Christmas lights with a Christmas tree light tester to find the bad bulb. Note that there are different models for LED versus incandescent lights, so be sure to purchase the correct tool based on the type of lights.

  • Start at one end of the strand or where the lights are not working. 

  • Follow the instructions on the tester to test every bulb, even the ones that appear to be working.

  • Discard any bulbs that don't work and replace them.

7. Use a Non-Contact Voltage Tester

A non-contact voltage tester works similarly to a Christmas light tester, as it checks for AC voltage in your Christmas lights. This method of testing Christmas lights makes it easier to identify a faulty bulb without the hassle of removing and replacing individual ones. 

  • Plug the lights in.

  • Choose a section halfway between the end of the malfunctioning section and the section that still works, and hold the probe of the tester near the wires in the male plug. 

  • If the tester beeps or flashes, it means that section works. 

  • Repeat until the device doesn't beep. 

  • The bulb next to the one that doesn't prompt a response might be the faulty one, so you can replace that bulb to see if the strand works.

8. Use the Replacement Bulb Method

man replacing bad bulbs on lights
Photo: Alexeg84 / iStock / Getty Images Plus / Getty Images

If all else fails and you don't have a Christmas light tester, you may have to test each bulb manually. 

  • Unplug the light strand.

  • Removing one bulb at a time and replacing it with a new bulb. 

  • After each replacement, plug in the lights to test them. 

  • Continue this process until the problem resolves.

  • If the lights still don’t work, your best bet is to get new ones.

Need professional help with your project?
Get quotes from top-rated pros.
The homeowners guide to holiday decorating services
From average costs to expert advice, get all the answers you need to get your job done.
House decorated for Christmas
Jul 26, 2024
Jul 26, 2024
Christmas light installation costs depend mostly on your home’s square footage, but our expert guide breaks down all the factors that influence what you’ll pay.
A Christmas tree in a bright room
Aug 7, 2024
Aug 7, 2024
If you're dreaming of rocking around your—professionally decorated—tree this season, this is how much a Christmas tree decorating service will cost you.
Two women hanging Christmas lights from the ceiling
Aug 26, 2024
Aug 26, 2024
Learn how to hang Christmas lights on your ceiling like a pro. This guide helps you buy the right supplies, plan your design, and enjoy the light show.
Coastal house decorated with Christmas lights
Aug 7, 2024
Aug 7, 2024
Wrapping lights on palm trees is a fun project that requires basic decoration skills. Read on to learn how to decorate a palm tree for Christmas.
A small toddler girl looking at Christmas tree indoors
Aug 19, 2024
Aug 19, 2024
Is there a difference between cool white vs. warm white Christmas lights? Yes, but you can’t go wrong with either option. Learn what’s different here.
man installing Christmas lights on roof
Aug 9, 2024
Aug 9, 2024
Getting your Christmas lights out of their boxes only to discover that half a strand is out is frustrating. Here’s how to fix Christmas lights and replace light bulbs.