Things You'll Need
Non-contact voltage detector
Test the non-contact voltage detector before using it by rubbing the tester end against your hair. The static electricity generated by your hair should set off the voltage detector. If the voltage detector does not respond, make sure the voltage detector is on and the replace the batteries if necessary. If a section of outside Christmas lights goes dark, use the non-contact voltage detector to find the source of the problem without taking down the entire light string. Be sure the power is on and the lights are plugged in securely before testing.
This process can be time consuming, particularly if the entire string is unlit.
Christmas lights are wired to stay lit, even if one of the bulbs burns out; but, this doesn't always happen. Sometimes a couple of bulbs or the entire light string doesn't light up when you plug the cord into a power outlet. When this happens, use a non-contact voltage detector to determine exactly which light bulb is faulty.
Plug the lights into an outlet. Sit on the floor near the outlet, so that you can easily access the lights from the outlet to the end of the string.
Turn on the non-contact voltage detector. Place the non-contact voltage detector near the light bulb. You do not have to touch the light bulb for the voltage detector to work. If the voltage detector lights up, an electrical current is present at that location and the bulb is good. If the voltage detector does not light up, that indicates a bad bulb.
Replace the bad bulb, and then try the lights. If the lights do not work test all of the bulbs and replace any bad ones. If this does not solve the problem, there may be a fault in the wiring of the light string.
Discard the string of lights if they are not working because of faulty wiring.
Transplanted Yankee Erin Watson-Price lives in Birmingham, Ala., and has been writing freelance articles since 1997. She worked as writer/co-editor for Coast to Coast Dachshund Rescue's newsletter, "The Long and the Short of It." In 2007 she obtained a certification as a copy editor. Watson-Price holds a Bachelor of Arts in creative writing from Southern Illinois University-Edwardsville.