How to Find the Bad Bulb on a String of Christmas Lights

If you're able to locate a bad bulb, just unseat the bulb, remove the bulb from the plastic base and replace it with a similar bulb. Reinstall it and see if it works. Sometimes it does, but usually it doesn't.

When an entire string of Christmas lights goes out, it is because that string is wired in series. When there is a break of any kind, whether it is a bad base, bulb or wire, the entire string goes out. Most newer light strings, however, feature lights that have a built-in shunt, which usually allows for the string to remain lit and only the bad bulb to be out. Sadly, this doesn't always work.

In most Christmas light strings, there are three wires running the length of the string. One of these wires is the hot, one is the neutral and one travels from light socket to light socket. When using the hum tracer, it is important to test only the wire that goes between the sockets. Just separate the wires a little to determine which wire is the socket wire.

With the string of lights plugged in, turn on the hum tracer and place it against the wire on one side of the socket (one wire enters and one wire leaves the socket). If it hums, then you have 120 volts traveling within that piece of wire. Then, test the wire on the other side of the socket. If it also hums, then that socket and bulb are both good.

If you come across a line that does not give off a hum, that is where the problem lies. Replace the bulb, and if that is the only problem on the string, it will light up. If you replace it and the device picks up the hum, but the string still doesn't light, then there are additional bulbs out on the string. Keep looking.