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How to Troubleshoot and Fix Christmas Lights

With an Analog or Digital Multimeter You Can Be Ready for the Holiday Season in No Time

© 2011 by Kelly R. Smith; all rights reserved; content may not be copied, rewritten, or republished without author's written permission.

A plethora of Christmas lights on the front lawn

A plethora of Christmas lights on the front lawn

This article was last revised on 11/02/20.

Each year after Christmas, people are quick to put away their Christmas decorations and often just throw them into a cupboard or basement to get rid of them quickly. But, when December rolls around again all the decorations have to be taken out again and checked for damage. This traditional ritual typically happens after Thanksgiving. But in this chaotic time of the COVID-19 pandemic, whatever fits your schedule is OK.

Christmas lights are fragile and can be prone to breakage. The bulbs are extremely light and can break far too easily to say nothing of mysteriously becoming tangled on their own, like the cords on you ear buds. How does that happen, anyway?

As Christmas lights, either the traditional variety or the newer, more efficient LED plug-in Christmas lights for outdoor and indoor decoration are wired in series; when one bulb blows the whole chain of bulbs goes out and it can be very difficult to find out which bulb is blown.

What You Need for Troubleshooting

All you will need for this method is a multimeter and this guide. Without this method people generally start by replacing the first bulb and then move on to replacing each bulb in turn until they find the bad actor.

If you have done this before, you know this is very time-consuming and very frustrating! If there ever was a recipe for procrastination, this is it! You'll be happy to know that this method is incredibly faster and all you need is a multimeter, a little time, and a steaming mug of coffee.

A Step By Step Process

The first step is to check that the fuse in the plug is intact. Take the fuse out of the plug and test with your multimeter. You can find an article on how to use a multimeter here. The next step is to check the first and last bulb to make sure that they are working as designed.

A digital multimeter with test wires, photo courtesy Alan Bridge To do this, remove the bulb from its holder and identify the two contacts. Connect the two points of the multimeter to the two contacts on the bulb and you should hear a buzz from the multimeter.

This confirms that the bulb is OK. If at any stage of this process you find that a bulb is not working, you can replace it and you will be finished.

Presuming the first and last bulb are working fine you can proceed to the second step. You need to look at the socket from where you removed the first bulb. You will see three wires. One that came from the plug (or previous socket), one that is going to the next socket, and one that bypasses the socket. At the final socket there will only be two wires.

These are the one that came from the previous socket and the other wire that bypassed the other light sockets and is going back to the plug. At the very first socket connect the probe of the multimeter to the contact of the wire going to the next socket.

At the last socket connect the probe to the contact of the wire coming from the previous socket. The multimeter should not buzz; it should not indicate continuity. This confirms that the faulty bulb is between the first and the last socket.

This Troubleshooting Method is so Much Quicker!

Usually people would check each bulb in turn but with this method we will divide the number of lights we are testing in two distinct sections each time we do a test. Remove the middle bulb of the string and test it with the multimeter. Then connect the first probe of the multimeter to the first socket in the string in the same way as in the last test.

Then connect the second probe to the socket in the middle of the string you just removed the bulb from. Connect it to the contact of the wire coming from the previous socket.

If the multimeter buzzes you know that the first half of your string of lights is working as designed, so the problem is obviously in the second half of the string. If it doesn’t buzz you know you need to concentrate on the second half.

If you had a string of 400 bulbs, you now know that the broken bulb is one of either the first 200 or the last 200, so you have halved the number of bulbs to be tested.

If you keep following this process you will reduce the number of potentially damaged bulbs to 100, 50, 25 and so on until you find the damaged bulb. This is a lot quicker than testing each of the 400 bulbs in turn! There is power in working methodically.

What If Two Bulbs Are Burned Out?

It is entirely possible, in some cases probable with older strings, that two bulbs could be gone. To identify this situation you can test both halves when you divide the string in two. For example, when you initially selected the middle bulb, you can test its connection to the first bulb and to the last bulb.

If the multimeter fails to buzz on both sides then you know there is a faulty bulb on both sides. You then need to divide again and test both sides of the new divide. When you test a section where you do get a positive result from the multimeter you know that all the bulbs in that section are working ok.

Hopefully this how-to article on troubleshooting and fixing Christmas lights has taken some hassle out of your holiday season! If so, I would appreciate you sharing this page with your friends. Thanks for visiting!

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About the Author:

Photo of Kelly R. SmithKelly R. Smith was a commercial carpenter for 20 years before returning to night school at the University of Houston where he earned a Bachelor’s Degree in Computer Science. After working at NASA for a few years, he went on to develop software for the transportation and financial and energy trading industries. He has been writing, in one capacity or another, since he could hold a pencil. As a freelance writer now, he specializes in producing articles and blog content for a variety of clients. His personal blog is at I Can Fix Up My Home Blog where he muses on many different topics.

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