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In a household or office, when one light does not work, the bulb usually needs to be changed. Occasionally that does not work. If a bulb that is changed with a known good bulb still doesn’t work, check the list below for possible reasons in order of probability.
Fluorescent tube lights can be very difficult to troubleshoot because, unlike CFLs, they have a separate ballast. When fluorescent tubes don’t light up, it could be a bad tube or ballast. If there are known good tubes of the same diameter in another fixture, install them in the fixture that does not light up. If it still doesn’t light up, then most likely the ballast is bad, or there is no power to the fixture. Less likely, there could be a loose wire connection in the fixture.
If the tube is darkened at one end, it is likely that the tube is bad. See picture below for examples. If you have a bad tube in fixtures with more than one tube, change them all. If one is bad, the other is usually about ready to fail. With certain older ballasts, one bad tube will cause a good one not to light up.
If a fixture has a two lamp magnetic ballast, and one tube is bad, the other tube will not light up. It is preferred to replace both tubes when one goes bad anyway.
Place the tip of a Fluke® voltage detector near each end of each bulb in the fixture with power turned on. If the tester lights up, the ballast is probably good. The Fluke® tester excels with this ballast test.
If a light fixture has a switch, it could have failed. Sometimes the tab inside the socket could be bent down or burned. If it is burned, you will be able to see the damage. If you think the tab is bent, turn off power to the socket. Using a needle nose pliers, pull out the tab a little bit.
If a light does not work and it has been replaced with a known good bulb, check to see if any circuit breakers or GFCI receptacles are tripped (bathroom light could be on the load side of a GFCI.
The best way to test a single pole light switch is to turn the switch off, remove the switch cover and place the tip of a voltage detector next to each of the two hot terminals of the switch. The tester should indicate voltage at one of these two terminals indicating there is voltage present at the switch.
If the tester indicates voltage present at one of the terminals, turn on the switch to see if voltage is present at the other terminal. If voltage is present at the other terminal, the switch is good. If voltage is not present at the other terminal, the switch is probably bad.
If the voltage detector can not reach the terminals while the switch is still in the electrical box, test a switch with a continuity tester. Turn off power to the switch, disconnect one hot terminal, check for continuity as you turn on the switch.
Bad Fluorescent Tube with a Burned End