How to Test Christmas Lights: 6 Methods to Try

The seemingly annual struggle with how to fix the Christmas lights can be prevented with testing. Learning how to test Christmas lights will ensure you only hang functioning lights on your tree this year.

Plugging in a strand only to find that all or none of it lights up can look like a daunting challenge. However, with a few simple checks and tests, you can get your Christmas joy up and running. Here's how to test Christmas lights regardless of the type of lights or the issue.  

Why Should I Test Christmas Lights?

One thing worse than discovering your Christmas light strand doesn't work is finding out after installing them. Testing Christmas lights before decorating takes just a few minutes and can save a lot of time after the decorating work is complete. Plus, it can help you narrow down your light collection to the working strands, helping to declutter.

How to Test Christmas Lights

Learning how to test Christmas lights wired in series is almost as easy as the testing itself. Follow these steps to stay safe and keep your lights working year after year. 

1. Inspect the Strand

The first step is to inspect each light strand. Each year, look for signs of damage, including frayed wires or cuts or cracks in the wire sheathing along the whole length of each strand. Also, look for broken bulbs, damaged sockets, and missing bulbs. 

If you find frayed wires, damaged sheathing, or cracked sockets, discard the entire strand and replace it for safety, regardless of whether or not the lights work.

2. Plug in the Strands

After inspecting and determining your light strands look good, plug in each strand one at a time. In a perfect world, all the lights will light up, and everybody is happy. In reality, you're likely to find a few dark bulbs or a strand or length of strand that doesn't work. Here's where further testing comes into play. 

3. Check the Fuses

Modern Christmas light strands include a fuse or two inside a small chamber located in the male plug at the strand's beginning. Unplug the strand, look for a tiny sliding door, pry it open with a small flat screwdriver or fingernail, and remove each fuse. In some cases, you can see a blown fuse by noticing a dark patch of the glass surface. 

If it's not apparent whether they are working, try replacing them and plug in the strand again. If the strand lights, you're done. If not, you can place the probes of a multimeter set to ohms (Ω) on either end of the fuse and test for continuity. The fuse is good if the reading displays a number other than zero. Replace the blown fuse if the reading is infinite, open or zero. 

4. Use a Christmas Light Tester

Christmas light testers are available in several varieties, and some have functions that help repair broken light strands. The type of Christmas light tester you need depends on the type of light strand. You may need more than one tester for your whole collection. 

Types of these tools range from contactless electrical proximity testers to devices that attach directly to the strand to diagnose and sometimes repair any problems. Follow the manufacturer's directions on the model you choose for the type of lights. 

5. Use a Multimeter

As the name implies, you can use a multimeter for many electrical tests and diagnostics. To test a strand of Christmas lights, using a multimeter is the best way to attack the problem of a non-working strand by eliminating possible causes. 

You'll use your multimeter to check for continuity. The location of the continuity break indicates the location of the bulb that's burnt out. To check for continuity in the entire strand, place one multimeter probe into a hole on the female plug side of the strand. Place the other on one of the male prongs. The strand contains at least one bad bulb if the meter reads infinite, open, or zero. 

The next step is to check multiple bulbs at a time. Remove two bulbs several sockets away from each other. Place one multimeter probe into one socket and the other into the second open one. Check for continuity as above. If the circuit is open, the bad bulb is somewhere between the two bulbs you removed. 

If that section checks out as okay, move onto the next section until you find the problem bulb section. Continue testing smaller sections until you pinpoint the culprit.  

6. Test the Lights Manually

If you don't have access to a multimeter or Christmas light tester, you can still test your lights. First, grab an extra replacement bulb and test that it works in a known working strand of lights. Replace each bulb on the strand one by one using that good bulb, plugging the strand in between swaps to see if it lights up. 

Inspect each bulb as you remove them. Look for signs of discoloration or, if you can see it, a missing filament inside. Look closely, as spotting the difference between good and bad bulbs can be tough. 

This testing method works well when just a single bulb is at fault. However, when multiple burnt-out bulbs are the problem, a thorough inspection and a little luck are your best options. 

How Do Christmas Lights Work?

There are multiple ways manufacturers make and configure Christmas light strands. Older Christmas light strands are notorious for the entire light strand failing if a single bulb goes out. In most modern Christmas light strands, a single bulb can go out and not affect others. Simply replace the burnt-out bulb. 

Wired-In Series

Christmas lights are wired in a couple of ways. The first way is in what is known as a series. In a series, a single wire carries electricity from bulb to bulb and then connects to a neutral wire that completes the circuit. In this scenario, if a bulb burns out, the circuit is broken, and none of the lights will work. 

Modern Christmas lights wired in a series include a shunt device that helps bypass a burnt-out bulb, and the remaining lights stay lit. It's not a perfect system, and sometimes the strand goes dark anyway in that situation, but it helps. 

Wired-In Parallel

Christmas lights with a parallel wiring configuration avoid the broken circuit problem, which is when one bulb fails. The parallel setup is most common in outdoor Christmas lights, but you can find some models of parallel indoor lights. 

Parallel wiring includes additional wires that carry current to each bulb individually. That means that if one bulb is missing or not working, the rest of the lights continue working. Repairing a non-working strand of parallel Christmas lights is a matter of replacing a burnt-out bulb, replacing a fuse, repairing broken or fraying wires, or fixing broken bulb sockets. 

How to Prevent Blown Christmas Lights

There are a few ways to prevent blown Christmas lights. However, no amount of prevention can eliminate the simple fact that bulbs wear out. Testing and repairing lights is an annual process regardless of how careful you are with your lights. 

The trick to prevention is avoiding placing unnecessary stress on the bulbs or wiring. Just a bulb knocked loose in storage can be hard to track down. Place your Holiday lights back into their original packaging for the best results. However, that option isn't always available. In that case, create a spool from a sturdy piece of cardboard and wrap the light strand around it for storage.  

DIY vs. Hiring a Pro

While Christmas light technology is always improving along with the tools for testing and keeping them running, DIY Christmas lighting can still be challenging. If you're more of a holiday enjoyer than a holiday DIYer—a professional Christmas light installer can tackle the hard work for you. At the same time, you can more easily enjoy your holiday season and perhaps a mug of eggnog.