Basic mini Christmas lights are typically three-wire strands. For families intending to use the Christmas lights year after year, troubleshooting these lights may be necessary. When an entire string of lights stops working or a section stops working, it's important to troubleshoot the issue in order to get the lights working again.
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Unravel the entire strand of lights. Do this in a space where you have plenty of room where the lights do not become tangled again while you work. It sometimes works best to sit down and begin by working on each light, one at a time, pulling them across your lap as you work. Plug the lights into an outlet. Unscrew the first bulb carefully (if the entire strand is not lighting up, you will have to start with the first light, but if only a section is unlit, begin with that section), setting the bulb aside. Screw the replacement bulb into the empty hole (if the strand does not light up, replace the new bulb with the old bulb and continue checking the remaining bulbs). Check all bulbs until you find the bulb that needs to be replaced.
Unplug the lights. Locate the tiny fuses (there should be two: one on each side of the AC line, inside the AC plug). The door will either slide out or will have to be pried away, depending on the set of lights you have. Swap out a fuse with a replacement fuse (this typically is included with the lights purchase). Plug in the lights to determine whether you replaced the right fuse. If the lights still do not light, try replacing the second fuse.
Keep the lights unplugged. Check each bulb to make sure it is secure in its hole. Starting with the first bulb, check whether you can wiggle the bulb or if it has come unscrewed. Carefully screw in any loose bulbs, making sure not to screw them in too tightly and that they are straight.
When purchasing Christmas lights, purchase at least one additional strand to use for replacement bulbs. It may be difficult to find replacement bulbs several years after you purchased your lights.
Never string together more than three strands of lights at one time to avoid blowing the fuses.
Do not touch the electrical component of the strings when the lights are plugged in.