It can be frustrating when your string of Christmas tree lights won't work because of one blown bulb or fuse. You can find the offensive bulb or fuse with a Christmas tree light tester, and replace it, saving you both time and money.
Insert the required number of batteries into the light tester and replace the cover. Hold down the "Test" button; if it lights up, your batteries are working.
Push the wire contacts of the bulb against the metal plates on the bulb tester. If it lights up, the bulb is good. If not, readjust it and try again. If it still does not light up, the bulb is bad and you should throw it away and replace it with a new bulb.
Read the instructions on bulb removal that came with your Christmas tree lights to find out if you have to unlock the bulb from the base before it can be removed. If so, unlock the bulb.
Insert the metal plate on the light test between the bulb and the base on your light set.
Twist the light tester gently left and right until the bulb is loose enough to pull out of the base with your hands. Remove the bulb from the base and insert a new one.
Test a fuse by removing it from your strand of Christmas lights and pressing it against the fuse tester part of your light tester. If the tester light does not come on, the fuse is bad and should be replaced.