An easy way to test a light fixture is to remove the bulb and replace it with one that you know is working. If you don't happen to have a working bulb handy, you can use a light socket tester or test the fixture with a multimeter. There are two procedures to check a light socket with a multimeter, one of which requires the power to be on and another you can do with the power off.
The first test is a voltage test, which you perform with the multimeter set to measure AC voltage in the 200-volt range. The second test is a continuity test to check the resistance between the light fixture terminals, and for this test, you set the multimeter to measure resistance. You perform a continuity test with the power off, but it isn't conclusive unless you disconnect the fixture.
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Every home should have a multimeter because it can help diagnose problems with all kinds of electrical fixtures, not just lightbulbs. A typical multimeter has a large number of dial settings, and if you're doing a light bulb socket voltage test, you set the meter to a voltage range suitable for household circuitry. Since residential circuits operate at 120 volts, the best setting is usually 200 volts.
Some multimeters distinguish between DC and AC voltage, with the latter being designated either as VAC or as V with a wavy line over it. You're measuring AC voltage, so choose VAC. After choosing the voltage setting, insert the black lead into the common port (COM) and the red lead into the mAVΩ port, remove the bulb, turn on the power and you're ready to test:
- Hold either lead on the metal socket casing.
- Touch the bulb at the bottom of the socket with the other lead.
- Record the reading. If it's somewhere close to 120 volts, the fixture is good. If you get a reading of zero or one significantly less than 100 volts, the fixture is bad.
If you get a very low reading, make sure the fixture isn't controlled by a dimmer. If it is, turn the dimmer up to full power and test again.
How to Test Continuity
Electricians use a continuity test to determine if there's a break in the circuit, so the fixture must be isolated from power to get a reading. If you have a plug-in lamp, you can just unplug it, but if you're testing a wall or ceiling fixture, you need to disconnect it from the circuit after first turning off the power.
A continuity test measures resistance (Ω), and an appropriate meter setting is a mid-range value, or about 2kΩ.
- Touch one lead to the ground screw or ground wire on the disconnected fixture.
- Touch the other wire to either the socket itself or to the bubble at the bottom of the socket. It's best to do two separate tests, one for the socket and one for the bulb at the bottom.
- Note the meter reading. If it's close to 0, the fixture is good. If the meter jumps all the way to the right or reads OL, which means open line, the fixture is bad.
The Light Socket Has Power but the Bulb Won't Light
If your voltage or continuity test shows that the fixture is working but the bulb won't light, try another bulb. If that doesn't work, the problem is corroded terminals. To clean them, you need to unplug the lamp or turn off the switch and the breaker.
Rub the inside of the socket with a rag or toothbrush soaked with isopropyl alcohol. If you can see visible rust, scrub it off with the toothbrush or use a small piece of sandpaper. Clean the bulb at the bottom of the socket the same way.