How to Fix Christmas Lights That Are Half Out: Step-by-Step Guide

Incandescent Christmas lights are hardwired in a series on two or more continuous circuits. When a bulb in the middle of the strand goes out, the circuit isn’t complete and the lights that follow won’t receive the proper voltage. LED lights are more durable, and when only part of a strand goes out, the issue most likely resides with one of the fuses. 

These devices typically cost between $25 and $30 and will make your job significantly easier if you need to track down one bad bulb on a strand. You can also test the bulbs with an electrician’s multimeter tool, but Christmas light testers include additional functions, like voltage detection, bulb removal, and shunt replacement. You don’t need a dedicated bulb testing device to diagnose and fix your faulty strand, but it could save you time and hassle.

Sometimes corrosion on the prongs of the electrical plug is the culprit for a partial outage. Since it’s the easiest cause to diagnose or rule out, it’s best to start here. If you see dark spots or debris on the prongs, try cleaning them so that they can conduct the proper voltage. Spray on a commercial cleaner that’s safe for electrical contact and scrub it off with a microbrush. 

Alternatively, you can rub the corrosion off with a piece of fine-grit sandpaper. Once you clean the visible accumulation, wipe down with a clean cloth to remove any sanding dust. If some of the lights are still not turning on after you plug it back in, move on to the next step.

With incandescent Christmas lights, a blown fuse usually results in the whole strand going out, but with LED lights, you can potentially fix it with a new fuse. Look for an indented compartment on the plastic casing for the plug on the prong end of the strand. Using a small screwdriver or your fingernail, slide it open toward the prongs. You’ll find two small fuses inside. Delicately lift each fuse from the bottom and pop them out. 

If the glass on either of the fuses looks stained or dark, it's likely blown. If both fuses appear normal, go ahead and replace the fuses. Ensure your new fuses are the same size and amperage as the previous ones. Luckily, nearly all of them are rated for 3 amps. Use gentle pressure to slide the new fuse into the proper compartment until it clicks into place. You may need a small tool, like the tip of a pen to safely insert it. Close the compartment and plug the lights back in to test them. If a section of the lights remains dark, move on to the next method.

If the fuses and prongs aren’t the problem, it’s safe to assume a bad bulb is the source of the issue. The first step toward locating the broken bulb is visually inspecting the whole strand. Look for bulbs with damage, have gone missing, and look unusual in some way. 

Another telltale sign of damage is silver- or black-colored staining on the inside of the bulb. Smoky-looking bulbs mean the amperage rating for the strand had been exceeded because it’s connected with too many other strands. Make sure to consult the rating on your set and avoid overloading the wire in the future. 

If your inspection hasn’t yielded any results, your next move is to check the strands for any loose bulbs. Plug the strand back in and, starting with the first unlit bulb, wiggle each one with your fingers or flick them gently with a pencil. If lights flicker when they move, it’s safe to assume it’s loose. Unplug the strand and gently press the bulb back in until it clicks into the socket. If that step has not resolved the issue after you plug the strand back in, you’ll need to replace the bulb.

After trying the other strategies, you have no choice but to check every bulb for damage. This task is significantly easier with use of a Christmas light bulb tester. These inexpensive tools often resemble a small pen. Touch the device to each bulb, and it will light up or beep when it reaches a malfunctioning bulb.

Before attempting to remove or add any bulbs, fully unplug the strand—otherwise, you risk electrocution and serious injury.

Some light strands feature a small plastic latch on the bulb seat. If that’s the case, lift the latch and leave it in the open position. Take the plastic seat of the bulb between your fingers and gently squeeze upward, lifting the seat and the bulb out of the socket together. Avoid pulling the bulb so you don’t damage the wires.

Replace the bulb with a spare from the kit or a new one from the same manufacturer. Trying to force a bulb onto a strand that doesn’t match can lead to a short and, in some cases, fire damage. Slide the new bulb into the socket and gently but firmly press it into place. Plug the string of lights in to confirm that the new bulb works and the connection is restored.

If the whole strand doesn’t light up after you’ve replaced the bad bulbs, the cause of the problem is likely a faulty shunt. The shunt is a small device that offers a low-resistance path for the electrical current to pass through the circuit and reach each bulb. 

You’ll need to use a Christmas light repair tool to repair a damaged shunt. Start by removing the bad bulb. Insert the socket into the repair tool (or vice-versa, depending on the model you’re using), and squeeze the trigger once or twice, per the manufacturer’s instructions. This action shoots a series of pulses through the circuit that should reactivate the shunt. Return the replacement bulb to the socket and plug the strand back in.