Black staining marks burning out or burned-out fluorescent light bulbs (C) Daniel Friedman at Fluorescent Bulb Troubleshooting
Diagnose & replace fluorescent light bulbs (lamps)

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Steps in diagnosing and repairing a flickering fluorescent light bulb or lamp:

When one or more fluorescent light fixtures are humming or flickering or if the light simply won't turn on, the problem may be something as simple as a bad fluorescent bulb.

Here we suggest the steps to follow to determine if the problem is a loose bulb or lamp, or a defective one that needs to be replaced.

Page top photo: blackening visible at one or both ends of a fluorescent bulb means that the bulb is at or near the end of its life.

This article series explains how to diagnose the causes of flickering, dim, or dead fluorescent lights at or in buildings.

Watch out: flickering or dimming lights might indicate a dangerous condition risking a building fire or an electrical shock. If the simple bulb or starter repairs we describe here don't cure the flickering, switch off the bad-acting light fixture, leave it off, circuit and ask for help from a licensed electrician.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Diagnose a Bad Fluorescent Light Bulb

Step 2 in installing a replacement fluorescent light bulb or lamp - rotate to snap into place (C) Daniel Friedman at InspectApedia.comIf the light fixture had trouble starting and kept flickering the problem could be a bad connector at one or both ends of the bulb, a loose bulb, or a bad bulb.

[Click to enlarge any image]

First check the bulb connections.

Taking care not to break the bulb and blind yourself with mercury1 and glass, gently grasp the fluorescent bulb and try turning it back and forth so that its connector pins scrape and clean their connections in the clips that connect to the bulb ends.

Sometimes I find that the bulb had never been seated properly in the first place, and on occasion I find that the connectors were dirty or arc-burned.

Rarely I find that the connector's internal metal spring clips have broken, or even fallen out of the connector completely so don't forget to try looking at the connector to see that its metal clips are inside.


Below on this page we show how to insert and twist the fluorescent lamp so that it snaps into place.

Next look at the bulb more-closely

If the bulb ends are blackened, even slightly, the bulb may be near the end of its life or even past it.

Burning out fluorescent  lamp (C) Daniel Friedman at

In my photo above the lit-bulb in this fluorescent fixture is quite black at one end.

Other bulbs, also blackened (shown at page top) refused to light at all. We replaced both the dead bulbs and also the nearly-dead one shown here.

Next try a replacement lamp

13-Watt F13 T5 Fluorescent Bulb Connector Pins (C) This is a GE F13 T5 13-watt mini bi-pin lamp

Take a bulb from a fixture working properly and test it in the flickering fixture; also look for black or other discoloration in the flickering bulb.

Often a failing fluorescent light bulb will begin to darken or even look black at one or both ends. Eventually the bulb simply won't light.

Replace it.

See REPLACE a FLUORESCENT LAMP / BULB just below on this page for details about how to remove and insert fluorescent lamps.

On a GE F13 / T5 Miniature Bi-Pin like the one shown above, F13 refers to a 13-watt fluorescent bulb, and T5 refers to the bulb shape (straight). This particular bulb is 20.91" long and its bulb diameter is 0.625 inches.

Watch out: handle fluorescent bulbs with care to avoid breaking one.

If you break one of these fluorescent light bulbs - containing mercury - you will release not only a million tiny fine glass fragments but also mercury laden dust that is toxic to breathe or get on your skin.

Get the right fluorescent bulb - or in electric-speak, "lamp" for your light fixture by taking the old one to your electrical or building supplier.

Finding the right bulb is easy if you have the old one, since that makes it easy to match the original by bulb type, size, and wattage.

At FLUORESCENT LIGHT BULB CODES & TYPES we provide a table of fluorescent bulb type and identification codes, and at FLUORESCENT LIGHT BULB PHOTOS we show you what these look like.

Some LED options and instructions for replacing a fluorescent bulb are at SOURCES for INSTRUCTIONS, PARTS, REPLACEMENTS for fluorescent bulbs, transformers, parts.

Dispose of the old fluorescent bulb properly

If you replace a fluorescent bulb, consider taking the old bulbs to your vendor for recycling.

In the U.S., accordance with the Resource Conservation & Recovery Act (RCRA), the mercury content in fluorescent lamps (bulbs) manufactured after 1990 was significantly-reduced; before 1990 these bulbs are higher in mercury content.

Manufacturers such as GE labeled the newer low-mercury bulbs with an "Eco" suffix.

Guidelines and regulations about disposing of fluorescent lamps are at FLUORESCENT LIGHT FIXTURE BULB / BALLAST DISPOSAL

What to check next

If replacement with a known-good lamp doesn't get the light fixture working the problem may be in the starter or in the ballast.

Try Replacing the Starter

The starter is a small canister that simply clips into a socket behind one end of the fluorescent lamp.


If necessary Replace the Ballast

If the problem is not the bulb, the starter, the connectors or wiring, and especially if the light fixture was humming the problem is probably the ballast.

Details on replacing and wiring a new transformer or ballast are at FLUORESCENT LIGHT TRANSFORMER / BALLAST REPLACEMENT.

How to Replace the Fluorescent Lamp / Bulb in the Fixture

How to insert a fluorescent bulb into the lamp sockets (C) Daniel Friedman at

[Click to enlarge any image]

Watch out: before you call for help because you can't find the starter on your fluorescent light, check the bulb and socket type Bi-pin bulbs and sockets support programmed start bulbs and require a separate starter.

Single center-pin type bulbs and sockets, also called instant-start bulbs, do not use a separate starter device.

Watch out: take care not to break the old or new bulb: you don't want a face of glass shards and toxic mercury. Wear eye protection.

Our photos should make clear what has to happen.

To remove the old bulb, turn it gently about 45 degrees to allow the bulb pins to align with the entry/exit slot in the bulb base and gently wiggle the bulb straight out.

To insert the new bulb reverse the process. Keep the bulb straight in line with the two bulb base sockets as you insert its pins straight into the socket slot, then rotate the bulb 45 degrees to feel it snap into place.

Step 2 in installing a replacement fluorescent light bulb or lamp - rotate to snap into place (C) Daniel Friedman at

The Leviton 13351 T8 / T12 turn-type lampholder shown in the photos above takes a 1" diameter bulb and is designed for a medium, bi-pin socket, "non-shunted" for programmed-start ballasts.

It is also rated for up to 660 Watts and 600 Volts - which will be far more than any typical residential fluorescent light bulb.

You can Also Replace Traditional Fluorescent Bulbs with LEDs

I've had good success replacing fluorescent bulbs with LED equivalents that use less power and are less annoying with flickering, starters, and transformers.

However some additional skill - or maybe less skill - is required in electrical wiring the replacement LED.

In short, you'll follow steps similar to these:

Watch out: be sure to read and follow the instructions from the LED replacement bulb manufacturer as the details will vary from what we show here.

Watch out: You could be shocked or killed. Turn off electrical power before touching or attempting to work on electrical lights or fixtures.

Watch out: if you are not trained and comfortable with proper electrical wiring and repair work, hire someone who is qualified. Making a mistake can cause a building fire, shock, injury, or death.

Turn all electrical power off

  1. Remove the fluorescent bulb
  2. Discharge the transformer by shorting it to ground
  3. You MIGHT need to remove the transformer (you may need to pay a disposal fee to get rid of the carcass)
  4. You MIGHT need to re-wire the lamp sockets to the incoming power wiring
  5. Install the LED bulb

GE's instructions add: Note: In case of fixture supplied with electronic ballast, the green cap of the GE LED T8 Universal tube can face any of the two directions. If tube does not start up properly, then you can try to exchange direction or go to the direct wiring/new luminaire section of the instructions. - source cited in detail below.

See some LED options and instructions for replacing a fluorescent bulb at SOURCES for INSTRUCTIONS, PARTS, REPLACEMENTS for fluorescent bulbs, transformers, parts


Continue reading at FLUORESCENT LIGHT BULB CODES & TYPES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see your local telephone listings for licensed electricians or see DIRECTORY OF ELECTRICIANS


Or see this

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DIAGNOSE a BAD FLUORESCENT LAMP / BULB at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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