A good fuse has an unbroken metal strip inside and shows near-zero resistance when tested with a multimeter. You can check most fuses in under a minute using nothing more than your eyes, though a quick electrical test removes all guesswork. Here’s how to do both.
Visual Inspection for Glass Fuses
Glass tube fuses are the easiest to check because you can see right through them. Inside every fuse is a thin metal strip (the element) that carries current. When a fuse blows, that strip melts and breaks, leaving a visible gap. In many cases you’ll also see a dark or metallic smear on the inside of the glass, left behind by the vaporized metal.
Hold the fuse up to a light source and look for three things: a clean, continuous wire running from one metal cap to the other, clear glass with no discoloration, and metal end caps that aren’t blackened or corroded. If the wire is intact and the glass is clear, the fuse is almost certainly good. If you see a gap in the wire, dark residue inside the tube, or any melting on the end caps, it’s blown.
When you’re unsure, grab a known working fuse of the same type and compare the two side by side. Differences in the wire or glass clarity become obvious.
Visual Inspection for Blade Fuses
Automotive blade fuses (the small plastic ones in your car’s fuse box) have a transparent or translucent housing. You can usually see the curved metal element through the top. A blown blade fuse will have a broken element with a visible gap, and the plastic may show burn marks, discoloration, or melting around the contact points. If the plastic housing looks warped or deformed, that signals a significant electrical overload.
Blade fuses are color-coded by amperage. Some common ratings: tan is 5A, red is 10A, blue is 15A, yellow is 20A, and green is 30A. Knowing this helps you confirm you’re looking at the right fuse for the circuit you’re troubleshooting, and it matters when it’s time to replace one.
When You Can’t See Inside
Ceramic cartridge fuses and some specialty fuses are completely opaque. No amount of squinting will tell you whether the internal element is intact. These fuses require an electrical test. Even with glass or plastic fuses, a visual check isn’t always conclusive. A fuse can look fine and still be degraded internally, reading high resistance without a clean break. If a circuit isn’t working and the fuse looks okay, test it electrically before moving on to other causes.
Testing With a Multimeter
A multimeter gives you a definitive answer. Before you start, always disconnect the equipment from power or pull the fuse out of its holder. Testing a fuse while it’s still in a live circuit can give misleading readings and poses a safety risk.
Set your multimeter to continuity mode (usually marked with a speaker or sound wave icon). Touch the two probes together first to confirm the meter beeps, which tells you it’s working. Then place one probe on each metal end of the fuse. If the meter beeps, the fuse is good. No beep means no continuous path for electricity, and the fuse is blown.
If your multimeter doesn’t have a continuity mode, use the resistance (ohms) setting instead. Touch the probes together to verify you get a reading at or near zero. Then test the fuse the same way. A good fuse reads between 0 and 5 ohms. A higher reading means the fuse is degraded. A reading of “OL” (over limit) or infinity means it’s completely blown.
Testing With an Automotive Test Light
If you’re checking fuses in a car and don’t have a multimeter handy, a simple test light works well, and you don’t even need to pull the fuses out. Clip the test light’s wire to a metal ground point on the chassis or the battery’s negative terminal. Then touch the probe to the small exposed metal contacts on top of each blade fuse, one side at a time.
A good fuse lights up the test light on both contacts, because current flows through the element from one side to the other. If the light only illuminates on one side, the internal connection is broken and the fuse is blown. This method lets you scan an entire fuse box in seconds without removing a single fuse.
Replacing a Fuse Safely
Always disconnect the device or turn off the vehicle before removing a fuse. Some fuse holders require a small screwdriver to unscrew a cap; many automotive fuse boxes include a small plastic puller tool clipped inside the lid.
The replacement fuse must match the original in both type and amperage rating. Using a higher-amp fuse won’t protect the circuit. Instead of the fuse blowing during an overload, the wiring itself can overheat, potentially causing a fire. The voltage rating matters too: always use a fuse rated at or above your system’s voltage. A fuse with too low a voltage rating can arc internally during a fault, which creates its own fire hazard.
Never substitute a fuse with aluminum foil, wire, or any other conductive material. This bypasses the protection entirely and can lead to electrical fires or electrocution. If a new fuse blows immediately or shortly after installation, the circuit has an underlying problem (a short circuit or an overloaded component) that needs to be diagnosed before you keep replacing fuses.