I bought one of these years ago and forgot how it works... this video is pretty good explaining it. In my own situation I had some outdoor strings that didn't function any longer, and I was able to fix them using this (after I replaced the battery). The use of the clamps to remember the "known good" part of the string is particularly helpful. Also the built in tester for the LEDs is really good because you can isolate an outage to either the LED or the socket. They can currently be gotten in the US for about $22. Given that, if you find a bad socket, the "fix" is to use one of their pods... (a clever idea) which basically replaces the socket with a small resistor, so you lose one light from the string. Also, a few sockets in my strings have two wires (an in and an out) and some have 3... (one is a through splice) and the pods are a lot harder to use on sockets that have 3 wires... since you have to twist two of the wires together and then get them both to fit through a small hole in the pod... which they won't... unless you strip the wires to expose copper in the hole... and a little outside which makes them less waterproof. A little bit of some sort of sealing glop would probably help here. The two I've replaced... both had 3 wires... they were on the ends of the strings next to the power connectors. Also, if you need more than the pods supplied with the tester (I think there were 2), you have to buy new ones - a pack of 4 for $10. I was glad to be able to get an old string going last night after replacing several LEDs and a water damaged socket... but I'm not sure it was worth all the work since new strings are so inexpensive nowadays.