You can test a fuse in under a minute using a visual check, a multimeter, or a test light. The method depends on the type of fuse and the tools you have on hand. A good fuse has a continuous metal strip inside and reads between 0 and 5 ohms on a multimeter. A blown fuse shows a broken filament, no continuity, or an “OL” (over limit) reading.
Start With a Visual Inspection
Many blown fuses are obvious to the naked eye, so check visually before reaching for any tools. What you’re looking for depends on the fuse type.
Glass tube fuses are the easiest to inspect. Hold the fuse up to a light source and look through the glass for the thin metal wire running from one metal cap to the other. If that wire is broken, the fuse is blown. Also look for dark, metallic residue coating the inside of the glass. A blackened or cloudy tube is a near-certain sign the fuse failed from a surge.
Blade fuses (the colored plastic type common in cars) have a visible metal strip along the top. Pull the fuse out with a fuse puller or needle-nose pliers and look at that strip. A gap or break means it’s blown. Melted plastic around the blade connections or burn marks on the colored body are also giveaways.
Cartridge fuses, often found in older home electrical panels and commercial equipment, have an opaque ceramic body that blocks your view of the internal element. You can’t confirm a cartridge fuse is blown by looking at it. Check for bulging, cracking, or burn marks at the metal end caps, but plan on using a multimeter for a definitive answer.
How to Test a Fuse With a Multimeter
A multimeter gives you a clear yes-or-no answer on any fuse type, including cartridge fuses you can’t see through. Here’s how to do it.
Set Up the Multimeter
Turn on your multimeter and rotate the dial to continuity mode, which is usually marked with a small speaker or sound wave icon. On many multimeters, continuity shares a position on the dial with the resistance (Ω) setting. Continuity mode is preferred because it gives you an audible beep rather than requiring you to read a number.
Before testing the fuse, verify the multimeter is working. Touch the red and black probes together. You should hear a beep in continuity mode, or see a reading at or near 0 ohms in resistance mode. If neither happens, check your battery or make sure the probes are plugged into the correct jacks.
Test the Fuse
Always disconnect the device or turn off the circuit before removing a fuse. Not doing so risks electrical shock. Once the fuse is out, place one probe on each metal end of the fuse. Press firmly to get solid contact with the terminals.
Read the Results
In continuity mode, a beep means the fuse is good. Silence means it’s blown.
If you’re using resistance mode instead, a reading between 0 and 5 ohms means the fuse is intact. Fuses have very little internal resistance, so any good fuse should read close to zero. A much higher number suggests a degraded fuse. A reading of “OL” (over limit) or “infinity” means there’s no electrical path at all, which confirms the fuse is blown.
Testing Car Fuses With a Test Light
If you’re checking fuses in a vehicle and don’t have a multimeter handy, a 12-volt test light works well. It’s actually faster for automotive work because you can test fuses without pulling them out of the fuse box.
Open your vehicle’s fuse box panel (check your owner’s manual for the location, as most cars have at least two). Clip the test light’s ground lead to the car battery’s negative terminal or any unpainted metal surface on the vehicle’s frame. Then touch the test light’s probe tip into one of the two small test points on top of the fuse. These are the round or square holes exposed on blade fuses even when they’re seated in the panel.
If the test light glows, that side of the fuse has power. Now poke the probe into the second hole. The two holes represent power in and power out, so both need to light up for the fuse to be working. If only one side lights up, the internal strip is broken and the fuse is blown. If neither side lights up, the fuse could be blown, or that circuit may simply not be powered at the time. You don’t need the engine running for this test, but the ignition may need to be in the “on” or “accessory” position for certain circuits to receive power.
What a Blown Fuse Tells You
A single blown fuse isn’t unusual. It may have simply reached the end of its life, or a momentary power surge popped it. Replace it with a fuse of the same amperage rating (printed on the fuse body) and move on.
A fuse that blows again right after replacement is a different story. That pattern points to an underlying problem in the circuit, and swapping in higher-amperage fuses to “fix” it is dangerous because it removes the protection the fuse provides. The most common causes of repeat failures are:
- Circuit overload: Too many devices drawing power from the same circuit, or a single device with a high startup current that exceeds the fuse’s rating.
- Short circuit: A live wire making direct contact with a neutral or ground wire, often caused by damaged insulation, pinched wiring, or moisture in the system.
- Faulty components: A failing motor, heating element, or fan can draw abnormal current that overwhelms the fuse.
- Wiring errors: Incorrect connections from a recent repair, modification, or installation.
If you replace a fuse and it blows immediately or within minutes, stop replacing fuses and start looking for the root cause. In a car, this often means a shorted wire rubbing against metal somewhere in the harness. In a home, it could be a failing appliance on that circuit. Unplug everything on the circuit, replace the fuse, and reconnect devices one at a time to isolate which one is causing the problem.
Quick Reference by Fuse Type
- Glass tube (home electronics, older equipment): Visual check first. Hold to light and look for a broken wire or dark residue. Confirm with a multimeter if unclear.
- Blade (automotive): Visual check or test light for speed. Look for a broken strip through the colored plastic, or probe both test points with a 12V test light.
- Cartridge (home panels, commercial): Skip the visual check. These require a multimeter. Look for physical damage like bulging or cracked ceramic, but test electrically for a definitive answer.
- Screw-in/Edison base (older home panels): Look through the glass window on the face for a broken wire or blackened glass. Check the screw threads for signs of overheating like discoloration or melting.