How to Replace a Bulb on a String of Christmas Lights

Lead Image
  • 1-2 hours
  • Beginner
  • 20-40
What You'll Need
Hum-tracing device

Who doesn't love a pretty string of lights? Christmas or otherwise, they're a great source of warm, diffused lighting, creating a cozy glow wherever they plug in. But what most of us don't like is finding light strings that don't work, and usually that's because of a single defective bulb. Many otherwise good light strings get discarded because we don't know how to find and change the one or two bulbs that ruin the entire line. Here's how to find the and swap out the culprits.

Step 1 - Locate Dark Bulbs

Finding the defective bulb in a string of Christmas tree lights is almost always the greatest challenge you'll have. It is not easy to identify the offending bulb. When a single bulb becomes defective it can extinguish all lights on the string. Some burned out bulbs become darker than the good bulbs, and occasionally you can identify the bad bulb as one that is darker, then replace it with a good bulb. But not always.

Step 2 - Locate Built-In Shunt Bulbs

Some of the newer light strings are made with a built-in shunt. In these strings one bulb can become defective without making all other bulbs stop working. If all bulbs on your light string are working except for one, it is likely that this string has the shunt feature built into the string. In this case, you can easily identify the bad bulb as the only one that is not lighted. Then, you can easily replace it by simply pulling it out of its socket.

Step 3 - Use a Hum Tracer

A hum tracer is one name for a device that can scan a line of lights to find a defective bulb. If your light string includes three strands—or wires—you should first identify which of the three strands (Strand A) runs from bulb to bulb. Strands B and C do not connect to each bulb. Because all three strands are usually twisted together it is sometimes difficult to tell which of them is Strand A. You may need to spread them apart to find the right strand.

Once you've identified Strand A, touch it with the tip of your hum tracer. If you hear an electronic hum, this means there is an electric current flowing through this strand. It is usually at a defective bulb that this current stops flowing. If you can locate a bulb where you get a hum on the incoming strand but not on the outgoing strand, this is an indication that the bulb you tested is defective.

Step 4 - Replace a Bad Bulb

Once you've identified a bulb as one that is probably bad, you can replace it with a new one. If the string lights come on, you have solved your problem. If the new light fails to come on, there are three possible reasons: 1) There is at least one other light on the string that is bad, 2) A wire may be severed or shorted, or 3) The new light bulb may be bad. If you cannot find another bad light bulb, you will likely need to replace your light string.