A strand that is half dead is not broken. It is open, and those are different words. Half the runs I pull off customer eaves have exactly one failure point: a dead LED that killed its series segment, or a cut wire at one spot. Everything else on the string is fine, and it stays fine if you find that one point. The strands that are truly dead are the ones with six dark gaps, and they are dead the same way.
The Test That Tells You What Died
Start with the boring list, because half of "dead" strings are one of these: the two fuses in the female connector are both blown (check both, they blow in pairs), a socket is corroded, or a bulb is loose and not seating. None of these need a tool beyond the spare fuse and a dry finger.
Now look at the pattern of dark LEDs, because that is the real diagnostic. A contiguous dark run with bright LEDs on both sides is a single failure inside that run: one LED, or one cut in the wire, with current stopped dead. Swap the suspect LED into a known-good socket and watch. The darkness moves with the bulb, and you have a dead LED. It stays where it is, and you have a cut wire. That swap test is the one repair step that almost nobody does wrong, and it takes a minute.
The Bypass Is Two Seconds of Work
The fix for a dead LED in a series run is a bypass: a short length of wire spanned across the two leads of the dead LED, closing the circuit around it. The math is why this works. A 100-LED series string draws on the order of 12 milliamps, and one LED is only a fraction of the string's total resistance. Bypass one and the current creeps up by 2 to 3 percent. The wire does not notice, the other LEDs do not notice, and the string runs at its rated brightness. The job is to keep that span clean: strip the leads, wrap a neat turn, solder if you can, and hold the span down to the socket body where it cannot chafe or pull loose in March.
A cut wire is the same idea, one cut shorter. Strip the two ends back to the bright copper, twist, and solder or tape. Do it at a socket, not mid-run, because the socket is where the wire flexes and where the repair will fail in a year if it is not held.
Where Repair Goes Wrong, and What to Buy Next
The mistakes I see on returned sets are predictable. People bypass three or four LEDs, which stacks past the string's rated current and the wire gets warm at the splice, then the next dark LED is blamed on the repair. People splice mid-run and leave the joint loose, and the first snow load breaks it at the cold spot. And people skip the swap test entirely, wire a bunch of spans, and hand back a strand that was simply missing one fuse.
For what you buy next time: the strand that survives a single failure is the one with individual bypass diodes already built into every socket, and that is a design choice you can read off the spec sheet before you pay. The [string lights range](https://lyxu.com/christmas lights/String lights) on the shop is where I keep the repairable sets, and the [christmas lights collection](https://lyxu.com/christmas lights) is where I send anyone who has given up on a strand and just wants the next season to stop being this conversation.