Corrosion on the plug prongs adds resistance that may prevent the necessary voltage from flowing through the light string. Once you clear the corrosion away, your lights may work again! If this doesn’t work, move on to other troubleshooting measures.
If the second string still works, then you likely have a problem with a bad bulb or fuse in the first string. If the fuse is good but the string is still not working it is usually a broken connection somewhere in the line. If the functioning string doesn’t work when plugged into the half-working string, there’s likely a problem with the wiring or, if you have incandescent bulbs, one or more shunts within the bulbs. [3] X Research source
Depending on your tester model, it may beep or light up when you find a bad bulb. If you find one bad bulb, keep checking for more. Mark each one with masking tape so you know which ones to replace. Move on to replacing any bad bulbs that you find. If you don’t find any bad bulbs, move on to checking and replacing the fuses. You can get a bulb tester anywhere holiday lights are sold. Consider buying a combination model that includes a tester and repair tool.
For your safety, always unplug the string of lights before replacing a bulb. Some bulb seats have plastic latches that snap onto the socket exterior. Simply lift up any latches before pulling out the bulb and seat.
When buying strings of lights, consider buying an extra string for every 3 or so that you buy—that is, buy 8 strings if you need 6. Use the extra string(s) simply for backup bulbs. Pull the bulbs from the extra string(s), put them in a labeled bag, and store the bulb-free string(s) for use if the wiring on one of your other strings goes bad. If you don’t have an exact match, don’t try to force a bulb that doesn’t completely fit into place. Non-matching bulbs are far more likely to fail and, in very rare cases, may present a fire hazard if they short. It’s safer to just buy replacement light strings.
If all the lights don’t come on, run the tester over your replacement bulb to make sure it is seated properly. If it isn’t, try removing and re-setting it again. If it is, move on to checking for other problems with the string of lights.
A small screwdriver comes in handy here, although you may be able to slide open the compartment with an opened paperclip or your fingernail.
Most string lights have a small fuse located in the plug end of the string. A blown fuse occurs when the small sacrificial wire within the fuse melts. It’s meant to do so as a protective measure if an excessive current passes through the line, although fuses can fail for other reasons as well.
Push your replacement fuses down into the compartment until they are fully seated. The tip of a pen may come in handy to help push the fuse in place. Slide the cover back over the compartment, then plug in the string and see if all the lights work. If replacing any bad bulbs and fuses hasn’t solved your problem, you likely have an issue with a bad shunt (in incandescent lights) or bad wiring.
A good light repair tool is multi-functional. It should act as a voltage detector, bulb tester, bulb remover, and shunt repairer. The best-known option is likely the LightKeeper Pro, which looks like a small plastic pistol.
You can remove any bulb in the darkened section, or even a lit bulb that’s near the darkened section. Unplug the light string before removing a bulb, though.
Follow the specific instructions for your light repair tool. You may be instructed to trigger several pulses through the light string. Shunts are small backup wires inside each incandescent bulb that are meant to take over if the filament in the bulb fails, thereby maintaining a completed circuit. The pulses from the light repair tool should be enough to melt the insulated coating and activate any shunts that aren’t working.
If the string still won’t light fully, it’s probably time to just replace the whole thing.
