Last night I had the good ole plug them in and 2/3 of the lights do not come on AFTER testing them in the garage. I have found out a couple of useful things. The strands that I am talking about are the C9-100LED's. 1) These starnds are actually split up into 3 sections and a bulb going out will kill that section of the strand. On the 1st section I checked 8 lights before finding the bad one the second was 15 before I found the culprit.
2) the Christmas Light Tester/Repair Gun can test a LED bulb BUT the bad part is that the buld socket is too big to fit inside the opening that over rides the strand and make the rest of the bulbs work.
I hope this helps.