The fastest way to test a fluorescent tube is with a multimeter set to continuity mode. Each end of the tube has two pins connected by a thin filament inside the glass. If either filament is broken, the tube is dead. The whole test takes about 30 seconds per tube once you know what to look for.
What You Need
A digital multimeter is the only tool required. Most models have a continuity setting marked with a small speaker icon, or you can use the lowest ohms (Ω) range. You’ll also need the tube itself, removed from the fixture. Always unplug the fixture from wall power before removing the tube.
How to Check Filament Continuity
A fluorescent tube has two pins sticking out of each end. Those pins connect to a small heating coil, called a filament, inside the tube. When you flip the light switch, the filament heats up and helps the gas inside the tube ionize and produce light. If the filament breaks, the tube can’t start.
Set your multimeter to continuity or low ohms. Touch one probe to each of the two pins on one end of the tube. A good filament will cause the meter to beep (in continuity mode) or show a reading near 0 ohms. Then repeat the same test on the other end. If either end gives you no beep and shows “OL” (open line) on the display, that filament is burned out and the tube needs to be replaced.
This test works for standard linear tubes (T8, T12, T5) and compact fluorescent bulbs. CFLs have the same internal filament structure, just in a smaller package with pins or a screw base.
When the Tube Passes but Still Won’t Light
A continuity test only confirms the filaments are intact. A tube can pass this test and still fail to produce light. Inside the glass, a small amount of mercury vapor and argon gas create the conditions for light when electricity arcs between the two ends. Over time, the gas can leak through cracked seals, the electrodes can deteriorate from normal wear, or the mercury can migrate unevenly inside the tube. Any of these problems prevents the tube from “striking,” which is the moment the gas ionizes and the arc forms.
Visual clues help here. Dark gray or black rings near either end of the tube indicate heavy electrode wear. This is normal aging, but once the darkening is severe, the tube is near end of life even if the filaments still have continuity. A tube that flickers constantly, glows only at one end, or takes an unusually long time to fully light up is also showing signs of gas or electrode problems. The only reliable fix at that point is replacement.
Rule Out the Starter and Ballast First
Before blaming the tube, it’s worth checking the other components in the fixture. Many older fluorescent fixtures use a small cylindrical starter that twists into a socket near one end of the tube. The starter’s job is to briefly send current through the filaments to heat them, then open the circuit so the arc can strike. A failed starter is one of the most common reasons a fluorescent light won’t turn on, and starters are cheap and easy to swap.
To test the starter, simply replace it with a known good one. They’re standardized by wattage rating and cost very little. If the light works with a new starter, you’ve found your problem.
The ballast is the heavier component inside the fixture that regulates current flow. A failed ballast can hum loudly, cause the tube to flicker, or prevent it from lighting at all. If you’ve confirmed the tube has good filaments and the starter is fine (or the fixture uses an electronic ballast with no separate starter), the ballast is the likely culprit. Ballast testing requires checking its output voltage, which is high enough to be dangerous. If you suspect the ballast, replacing the entire fixture is often more practical than diagnosing it, especially since LED retrofit tubes can bypass the ballast entirely.
A Quick Swap Test
If you don’t have a multimeter, the simplest approach is a swap test. Take the tube you’re questioning and install it in a fixture you know works. If it lights up normally, your tube is fine and the problem is in the original fixture. If it still won’t light, the tube is likely the issue. This method catches both filament failures and gas problems that a continuity test alone would miss.
Handling a Broken Tube Safely
Fluorescent tubes contain a small amount of mercury sealed inside the glass. If a tube breaks during removal or testing, mercury vapor is released into the air and small droplets of liquid mercury fall to the ground, where they continue to evaporate. The primary exposure risk is breathing in the vapor.
If a tube breaks indoors, open windows to ventilate the area and leave the room for 10 to 15 minutes. Do not sweep up the broken glass with a broom, which spreads mercury into smaller droplets. Do not use a regular vacuum cleaner, which will blow mercury vapor into the air and contaminate the machine. Instead, use stiff cardboard or paper to scoop up the larger pieces, then press sticky tape against the area to pick up smaller fragments and powder. Seal everything in a plastic bag.
Disposing of Old Tubes
Because of the mercury content, fluorescent tubes should not go into regular household trash if you can avoid it. The EPA recommends recycling them through local hazardous waste collection programs. Many hardware stores accept used fluorescent tubes at no charge. At least six states, including California, Massachusetts, Vermont, and Washington, prohibit mercury-containing lamps from going into landfills entirely. If your area does allow regular trash disposal, seal the tube in a plastic bag before putting it in your outside bin.