Before you panic, we have good news. This little speed bump does not mean your Christmas lights are destined for the trash or that you're destined to be without holiday decor and known as the neighborhood Scrooge. With a few easy-to-follow steps, you can save your strand of lights for yet another year of holiday cheer.
Note that if you have very old light strings, they may use older-style screw-in incandescent bulbs, which can be difficult (and costly) to repair. Truth be told, these older strings may not be worth fixing—the low cost of more modern bulb strings (which are either small incandescent mini lights or the more modern form, LED lights) makes buying a new set almost a no-brainer.
Check the Fuses
If one of your light strings isn’t working at all, or if only half of the lights are working, unplug the string and look for a small access door on one side of the male plug (the one with the prongs). If you find one, open it and inspect the very small fuses inside the plug. There should be two.
If one or both of these fuses look black or cloudy, or if you can see that the wire through the fuse has burned in two, you'll need to replace the ones that look bad. Even if you don’t have spare fuses on-hand, look at the old one for a size marking. Three-amp, five-amp, and seven-amp fuses are the most common sizes and you can typically find replacements at most hardware stores.
There's a good chance that replacing the damaged or burnt-out fuses will solve your lighting issue. If you do so and find that your lights still aren't working, move on to step two.
Check for Damaged Bulbs
If the fuses appear to be okay (or if your light set doesn’t have fuses from what you can tell) and some of the lights still aren’t working, you can start checking the bulbs for issues. Unlike the regular lights and receptacles in your house, light strings—especially older ones—are often constructed with the light bulbs wired in series. This means that power goes through each light bulb to get to the next one. If one fails, burns out, or is broken, the power will often stop there, resulting in a dark strand of lights beyond that point.
Unfortunately, finding the bulb responsible for the outage isn't always easy. While time-consuming, your first option is to comb the strand for any indications as to which light may be faulty. One-by-one, inspect each bulb as you move from one end of the strand to the other, observing whether a bulb is loose (if so, tighten it), broken, or burnt-out (often indicated by blackness in the tip).
If you're unable to locate the issue this way, your next step should be to use an electrician's multimeter or other tool specifically designed to test voltage. This will allow you to determine where the flow of electricity is stopping along the strand, which is a good indicator as to which bulb is to blame.
Replace Broken Bulbs
Should you find any bulbs exhibiting these issues, remove them and replace them with a new bulb, which can be found any the hardware store (some may have even come with your strand when initially purchased, so it's a good idea to keep those on-hand). To do so, grip the plastic base of the bulb and pull it out of the socket carefully. Look at the bottom of the socket—you should see two fine, flat wires, resembling small, stiff pieces of ribbon. Straighten those out and inspect them for corrosion. If they are in otherwise good shape, take a new bulb and align the two wires in the base with the two slots in the socket, then push in the new light bulb until it is firmly set into the socket.
Store Your Strands Safely
To save yourself the trouble next year, pack away your light strands safely once the holidays have passed. This means coiling them up without tangling, wrapping them in padding, and generally treating them with care so as not to break or damage any bulbs while in storage.
Switch Up Your Set
If you haven't already, consider upgrading to LED Christmas light strands next holiday season. While they may cost more initially, they'll last longer (and need fewer repairs) than incandescent lights, which will save you money and time in the long run. Plus, they don't get hot to the touch light other varieties of holiday lights do, meaning you can use them anywhere around your home stress-free.