How to Check if the Ballast is Bad in a Fluorescent Light? – 3 Ways


Written by

Thomas Carter



Marcos E. Williamson

how to check if the ballast is bad in a fluorescent light

When your fixture starts acting out by not turning on, changing colors, dimming, or flickering, it is because you have a bad bulb or a bad ballast. If it is a ballast problem, you will have to get a new one for replacement.

So, how to check if the ballast is bad in a fluorescent light?

We will show you three ways in this blog post. You can choose one to check fluorescent light ballast or try all three to test a fluorescent light fixture.

Read until the end for a complete understanding!

How to Tell if Fluorescent Bulb or Ballast is Bad?

Way #1 – Check For Bad Ballast Symptoms


Fluorescent light not working, fluorescent light dim, not bright, or not fully lighting can be signs of a bad fluorescent bulb or ballast. Therefore, it is hard to pinpoint the ballast as the problem.

To be more certain, you should look for specific symptoms of a bad ballast.

Locate it in your fixture and expose it for a closer physical examination. In most light fixtures, the ballast will be within a casing or housing that needs to be unscrewed. Be sure to enlist the help of appropriate tools if that is the case.

  • The ballast is problematic if its casing or housing is deformed or damaged. A common instance is it appearing swollen or as if it is about to blow up. You can take it as an indicator it is driven to overload. (In this case, you might want to check ballast output voltage).
  • Another sign is if it has burn marks across its body or wiring. This means that it has overheated and overloaded. It can no longer regulate the current to function properly.
  • If there is moisture or oil leaks inside the ballast, you can also presume that it is bad or about to go bad. Moisture and oil are among the most common electronic ballast failure causes.

You can be sure that your ballast is fried and needs to be replaced instantly. It is what is causing your fixture to experience problems, like delay starts.

Way #2 – Change The Bulb


The easiest way to tell if fluorescent lights slow to come on or still not working is due to a bad ballast is to change the bulb.

If the fixture’s problems remain after you have switched the bulb, you can conclude that the issue is with your ballast, not your bulb.

  1. Make sure you switch the power off before you proceed with your removal process. This ensures that you will not electrocute yourself.
  2. Before grabbing a bulb, test-touch it with the back of your hand. It should be cool.
  3. Simply turn it counterclockwise until you can pull it loose. Do not forget to support it with one hand to keep it from dropping and breaking into pieces.
  4. Put the new one in, put the fixture cover back, and switch the power on to check the results. If it lights up well, the problem is not a bad ballast, but a bad bulb.

Way #3 – Test The Ballast With A Multimeter/With An Electrical Meter


A multimeter can tell you whether your ballast is faulty, and using it to do so is relatively simple. Here is how to test a fluorescent light ballast with a multimeter:

  1. Switch off the power so that you will not risk electrocuting yourself.
  2. Detach the cover of the light fixture, then remove the bulb to access the ballast.
  3. Remove the casing or housing of the ballast to expose it.
  4. Grab the multimeter and put it on its highest resistance setting.
  5. Hold the black probe to the ground wire, and at the same time the red probe to the other wires.
  6. Get the reading. If it displays “OL” short for Open Loop, your ballast is not bad.

It might also read “1”, which indicates that the ballast is good as well.

What Happens When a Ballast Goes Bad? Is a Bad Ballast Dangerous?

A bad ballast can be dangerous if left unaddressed in fluorescent fixtures. A ballast is essentially a component that regulates the amount of electrical current that flows through a lighting system.

  • If this part is damaged or malfunctioning, it can cause flickering or dimming of lights, and even complete shutdowns, which may dangerously interfere with visibility.
  • In some cases, it may also emit electrical surges and sparks that could start fires.
  • Furthermore, faulty ballasts could lead to damage and failure of other components within the fixture, posing additional risks.

Therefore, it is critical to have any issues with your ballast resolved promptly to ensure both safety and optimal performance from your lighting system.

Frequently Asked Questions (FAQs)


What is the lifespan of a ballast?

A ballast can last up to 20 years if it is well-maintained. Electric ballasts tend to last longer than magnetic ones, with the latter’s service life ranging from 12 to 15 years.

Ballasts installed in the ideal environment with stable temperatures, low humidity, little dust, etc., will be able to serve longer. Other factors like the quality and brand of the ballast and how often it is used also plays a role in how long it lasts.

Can you use a fluorescent bulb without a ballast?

Fluorescent light bulbs count on ballasts to operate properly. The ballast manages the amount of electricity that flows through the bulb, ensuring stable operation and extending its lifespan. You can use other types of bulbs without a ballast by rewiring it.

This involves bypassing the ballast and connecting the bulb directly to a power source. However, if you intend to use bulbs without a ballast, proceed with caution and consult an electrician if necessary.

How to tell if T8 ballast is bad?

The signs that you have a bad T8 or T12 ballast are:

  • Buzzing sounds
  • Flickering and strobing lights
  • Taking longer to turn on
  • Dim lights
  • Fading coloration on the lamp


How to check if the ballast is bad in a fluorescent light? As you have read, you can check for physical symptoms and test a ballast by changing the bulb or with a multimeter.

The ways to test fluorescent light ballasts that we have detailed above can help recognize and fix the issues. So, regardless of the type you have, you can apply what you have gotten from this blog post.

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