How to Tell If Fluorescent Bulb or Ballast Is Bad-The Best Guide

Last updated on October 17th, 2023 at 09:08 pm

This article is a helpful guide for you. I would explain how to tell if a fluorescent bulb or the ballast is bad. Although fluorescent lights are outdated yet still some people use them.

I still have a few of them in my home. I have dealt with fluorescent issues in the past many years. Therefore, I have the necessary knowledge and experience.

I feel glad to share my knowledge with you. Hopefully, it will be very useful for my readers.

What is a Ballast?

Before moving further, let us have a brief overview of what is a ballast and how it works. A ballast is a kind of regulator. It monitors the lightning mechanism, control, and stabilize the light output.

It is a very important part of fluorescent and HID bulbs. It can be used with a dimmer to regulate the light output of the lamp.

In short, ballast is like the heart of the lights. Ballasts are available in two types; magnetic and electronic.

Signs of a Bad Ballast

Certain signs indicate that the ballast is bad. If you observe your light bulbs carefully, you can notice the following things;


Periodical flickering of bulbs indicates a bad ballast. Sometimes, flickering occurs when the bulb is about to die. But, it is not the case always.


A buzzing or humming sound is also a sign of a bad ballast. Sometimes, it gives off cracking noises.

How to Tell If Fluorescent Bulb or Ballast Is Bad-The Best Guide

Abnormal Start

The lights should be fully bright as soon as you turn them ON. A bad ballast causes the lights to act like HID. They attain full brightness very slowly. It is due to a bad ballast.

Low Brightness

Low lumen output is another indicator of bad ballast in your fluorescent bulbs. However, it can be due to aging or an old lamp.

Varying Light Output

Over time, fluorescent lamps start dimming. They fade or show color variations. Observe your bulbs and look for dark corners. You will notice inconsistency in the light output. Some areas are totally dark and do not glow.

How to Confirm Ballast or Bulb is Bad?

You can perform some simple tests to confirm whether the bulb or the blast is bad.

#1. The Multimeter Test

The straightforward way to test is by using a multimeter. Check the voltage output of your ballast. Operating a multimeter is very easy. They are available both in digital and analog types.

It has two probes, touch one of them with the hot wire and the other with the neutral wire. The needle of the analog multimeter should sweep to the right on the measuring scale if the ballast is alright.

The Digital meter will show “1” if there is no resistance. You can learn to operate the multimeter in this video.

#2. The Replacement Test

Another quick and easy way to confirm is by replacing. It is a trial and error method. Hold a brand new bulb and install it in place of the old one.

If the new bulb does not light up, it is sure that the ballast is bad. However, if the new bulbs work fully, the old one has failed.

Why Ballast of Fluorescent Bulb Fails?

The surrounding environment is the immediate cause of ballast failure. Here are some other common reasons behind premature ballast failure;

  • Overheating can damage your lights. Especially direct exposure to sunlight or a heat source.
  • Moreover, keeping the bulbs ON for an extended period also damages them.
  • Moisture is also an enemy of your bulbs. It can be due to rain or extremely moist air. Moisture also causes corrosion of the internal parts.
  • Likewise, too cold environment can also lead to issues. Because the device cannot work properly due to low temperature.

How to Replace a Ballast?

When it is confirmed that the ballast has failed, the only solution is to replace it. Kindly purchase a high-quality product from a trusted supplier. Here is how to do it;

  1. Turn OFF the electric supply to the fixture. Be sure to unplug the switch from the outlet.
  2. Remove the bulb from the fixture. Take off the cover of the ballast. Some are held by bolts, unscrew them.
  3. Snip the wires connected to the bad ballast. You have to snip several inches away from the end. Use pliers or wire cutters.
  4. Now, take out the ballast from its mounting position.
  5. Take the old one with you to purchase a compatible new ballast. Because they have different sorts of wiring, voltage, and current.
  6. Do not forget to compare the cost of the new fixture and new ballast. Purchase the fixture instead if the cost is too high.
  7. Follow the same directions to install the new one. Connect the wires, place the screws, and close the device.

How to Choose the Right Type of Ballast?

Compatibility is the most important factor you should consider. The ballast should be compatible with the fixture. Moreover, check the lamp/bulb type and line voltage.

This information is written on the label attached to the ballast. The common types of fluorescent bulbs are T12, T8, and T5. The symbol “T” refers to tubular and the number indicates the diameter in 1/8 of an inch.

Related: Methods to Connect LED Lights to Remote 

FAQs About Fluorescent Bulbs and Ballasts

If you have further queries, read the following commonly asked questions.

Can I replace a magnetic ballast with an electronic one?

Yes, you can install an electronic ballast in place of the magnetic one. Electric ones are good and energy efficient. While magnetic ones are hazardous due to harmful substances.

How long fluorescent ballasts can last?

Normally, a good ballast will last for 20 years. However, some external or internal factors can affect its lifespan. For example, a cold or hot environment decreases efficiency. Moreover, careless usage and poor maintenance of the bulbs also cause premature failure.

Can I put LED bulbs in fluorescent ballast?

Plug-and-play LED bulbs are compatible with fluorescent ballasts. So, you can replace them with LEDs. Just remove the fluorescent bulb and install the LED. There is no need to modify anything.

How to dispose of fluorescent bulbs?

Fluorescent bulbs contain mercury and can be harmful. You should dispose of a broken or bad bulb carefully. Wrap and seal it in a plastic bag. Put in the waste box for normal trash collection. It is better to take the broken or expired bulbs to the household hazardous waste collection center in your region.

Also Read: Best Solutions to Reset Day better LED Lights

Final Thoughts

After reading this detailed guide, you can now tell whether the Fluorescent bulb or the ballast is bad. Moreover, you have also learned the causes and symptoms of failure. I have also explained the method to replace and purchase a new ballast.