How to Fix Christmas Lights With a Multimeter

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Although string lights have many uses throughout the year, many people keep dedicated sets in storage to use exclusively at Christmas to adorn their Christmas tree, illuminate their home, and create lighted nativity scenes. For some reason known only to the Christmas angels — and perhaps the light manufacturers — string lights hate to be in storage and often refuse to work even though they were working when you put them in storage. Most of the time, the problem is caused by a single bulb, and depending on how the string is wired, that one bulb can make an entire section of bulbs or the whole string go out.


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These string lights had one job to do, and the most common response to their refusal to do it right is to throw the entire string in the trash and buy a new set of lights. That's a little hasty as well as wasteful and unnecessary, because you can use a multimeter to find the defective bulb, and the bulb is easy to replace. Sometimes, the problem is a blown fuse (which is inserted between the prongs of the male plug), and that's also easy to diagnose and replace.


Ready for the ultimate holiday hack? Here's how to fix your Christmas lights using a multimeter.

Checking Continuity With a Multimeter

It's fairly easy to check Christmas lights by doing a voltage test with a noncontact voltage tester, but you can't do that with a multimeter because the probes have to be in contact with the wires to get a reading. You can, however, use a multimeter to check continuity, which is a measure of resistance, and you can do this with the string unplugged, which is safer than testing the string when it's plugged in.


The first step is to check the resistance from one end of the string to the other. Like any electric circuit, the circuit in a set of string lights consists of a hot leg that runs through all the bulbs and a return leg that extends from the male plug on one end of the string (the part with the prongs) to the female plug on the other end (the part that looks like an outlet) and bypasses the bulbs. The return leg should be intact and show low resistance. If it doesn't, the wires are damaged, and although you could theoretically fix them, it's probably best to throw away the string.


You can also do a continuity test on the individual bulbs until you find the bad one. This is a bit of a painstaking process because you have to go from bulb to bulb. There's a more efficient way to find the bulb when you have a noncontact voltage tester, but since all you have is a multimeter, this process may take a few minutes.


Check the Entire String for Continuity

Step 1: Set Up the Multimeter

Adjust the dial on the multimeter so that the arrow points to ohms (Ω) of resistance. Plug the black lead into the common (COM) socket and the red lead into the volts-ohms (VΩ) socket. Touch the leads together. Since there is nothing between them, the resistance reading should be zero.


Step 2: Check the String for Continuity

Unplug the lights and insert one of the probes in the neutral slot of the female socket. (On two-prong polarized plugs, the neutral slot is the larger one.) If the plugs are polarized and the prongs are the same size, insert the probe into either slot. Now, touch the other probe to the larger of the prongs on the male plug. If the meter reads zero, meaning low resistance, the neutral leg is intact, and the string is good. If both prongs are the same size, test both of them, and if neither gives a zero reading, switch the other probe to the other slot on the female plug and try both prongs again. Any of the four combinations that produces a zero reading indicates the string is good.


Step 3: Check the Fuse and Replace It if Necessary

If you don't get a zero reading, check the fuse in the male plug by pulling the prongs out of the housing. The fuse is nestled between the prongs and is either a 3-amp or 5-amp one depending on the string. If the fuse has blackened or the filament inside — which looks like a light bulb filament — is broken (you'll be able to see this because the casing is clear glass) replace the fuse with a new one and check the continuity of the string again. If you still can't get a zero reading, it's time to replace the string.


Test for Bad Bulbs

Step 1: Start at One End of the String

Work systematically when testing bulbs to make sure you don't miss one. The best way to do this is to start at one end of the string and test each bulb in sequence.

Step 2: Pull Out the Bulb and Separate the Wires

Grasp the bulb with one hand and the socket with the other and pull sharply to remove the bulb. You'll see two wires extending out from the bottom and folded up along the side of the bulb. Separate these wires far enough apart to allow you to touch each with one of the probes without having the probes touch each other.

Step 3: Test for Continuity

Touch one probe to one of the wires on the bulb and the other probe to the other wire and check the meter. If it reads zero or close to it, the bulb is good. If you get a high resistance reading (denoted by OL, or Open Line on digital meters), the bulb is bad. Replace it.