Before doing this confirm that the second string is fully functional by plugging it into the outlet on its own.
Christmas tree lights half string out.
Incandescent christmas tree lights aka mini lights are wired in series so if your lights go partially out you have one of three issues.
For light strings that commonly have over 50 bulbs they are constructed in 2 or more continuous circuits.
A light bulb has come out of its socket or is half out of its socket and it has brought down the circuit.
Check the entire strand of lights for a missing damaged or abnormal looking bulb according to the christmas.
Its usually s a split string with half the lights on series on one leg and half the lights in series on the other leg.
One series leg has a bad bulb that took out the rest.
Watch as lou manfredini ace s home expert shows you how to troubleshoot if your christmas lights are half out.
How to fix half a string of christmas lights that are out step 1.
Plug a light string that works into the half out string.
Learn what to check and how to fix it.
If you can find it in 5 minutes or less fix it by using a replacement bulb cannibalized or a spare otherwise throw it away and buy a new one.