Ask me what kills a run of LED christmas string lights and I will not say the chip. I say the base. The little bi-pin socket is where the wire meets the bulb, and it is the one part nobody prints on the box. Most of the dead strings I get at a repair bench died at that joint, not at the LED.
G4 is a push-in, GU10 is a twist
A G4 is a 4 millimetre globe with straight pins that you push in. No turning, no clicking, the classic low-voltage base for fairy and string lights. A GU10 is a 10 millimetre base with a bayonet lock: you seat it and give it a quarter turn so the flanges catch. The "U" in the code is the giveaway, it means twist-lock, not push-in.
Put a GU10 in front of a G4 socket and it will not go. The globe is wider than the socket throat and the pins sit further apart than the holes. The reverse is worse, a G4 falls straight through a GU10 holder. I once spent forty minutes on a customer call before I read the base code off the back of the socket, and that one read saved the whole call.
The failure is the contact, not the glass
The socket crimps. Over a season of heat and cold a bulb that was only just holding will ratchet loose. The spring contact loses pressure, that one bulb flickers, then goes dark. In a series segment it drags the whole run down with it, which is exactly why people are sure the LED died when it was the base.
The fix is mechanical. A socket that clamps with a small ratchet and keeps the pin seated, and a bulb that is the right base for that socket. A G4 that is even slightly under-tensioned in the holder is the bulb that will be dark by the third cold snap of the season.
Pick the base before you pick the bulb
When you are specifying a run for a tree, an eave or a garland, decide the base first and buy the string to match. For a dense, fine fairy-light look the G4 mini is the workhorse, and a 300 LED Fairy String Light set (98ft) is the size I reach for when a customer wants coverage rather than one accent line.
For the tree and a denser finish, the 300 LED Christmas Tree String Lights is the piece I keep on the shelf. When the run is a long eave or an outdoor chain and the socket has to hold under wind and rain, a weatherproof 500 LED Outdoor Christmas Chain Light is a different tool from a plug-in indoor set.
And if you are replacing bulbs on a run you already have, read the base code off the socket before you leave the shop. GU10 is a downlight base from the halogen era, not a string-light base, and it will never seat in a G4 holder. The base decides what you can fix; the LED is the easy part.
Frequently asked
Are G4 and GU10 christmas string lights bulbs interchangeable? No. G4 is a 4mm push-in base and GU10 is a 10mm twist-lock base. The globe and pins are the wrong size for each other's socket, so they cannot be swapped.
Why does one LED go dark on my led christmas lights while the rest stay on? Usually a loose base contact at one socket, not a dead LED. The bulb ratchets out, the spring loses pressure, and that point drops out.
How do I tell which base my christmas string lights use? Read the code on the bulb or the socket. Straight pins with roughly 4mm spacing is a G base, push-in. A quarter-turn lock is a GU base.