A bad fuse has a broken metal strip inside it, and you can usually spot one just by looking at it. If the thin wire connecting the two metal prongs is snapped, melted, or blackened, the fuse is blown. For fuses you can’t see through, a quick test with a multimeter or test light will give you a definitive answer in seconds.
Check the Fuse Visually First
Most automotive blade fuses and many household plug fuses have a transparent or semi-transparent plastic body. Pull the fuse out and hold it up to a light source. You’re looking at the thin metal filament that bridges the two terminals inside. If that filament is continuous and unbroken, the fuse is fine. If it’s snapped in the middle, shows signs of melting, or has black or brown burn marks inside or on the outside of the plastic, the fuse has blown.
Sometimes the damage is obvious: a dark, scorched smear inside the casing. Other times the break in the filament is tiny and hard to see. If you’re squinting and still not sure, move on to testing with a tool.
Test With a Multimeter
A multimeter removes all guesswork. Set it to continuity mode (the symbol that looks like a sound wave or a small dot with curved lines). Before testing the fuse, touch the two probes together. You should hear a beep, which confirms the meter is working. Then place one probe on each end of the fuse, touching the metal contacts.
If the multimeter beeps, the fuse is good. Current can flow through it. If you get silence, the internal filament is broken and the fuse needs to be replaced.
You can also use resistance mode (ohms, shown as the Ω symbol). A good fuse reads at or very near zero ohms, meaning electricity passes through with almost no resistance. A blown fuse reads “OL” or infinity, which means the circuit is completely open and no current can pass. Either testing mode works, but continuity is faster since you just listen for the beep instead of reading a number.
Test With a Test Light (No Multimeter Needed)
If you’re checking fuses in a car and don’t have a multimeter handy, a simple circuit test light works well. You don’t even need to pull the fuses out. Each blade fuse in your vehicle’s fuse box has two small exposed metal test points on top.
Clip the test light’s ground lead to a bare metal surface on the vehicle’s frame or body. Then touch the test light’s probe to one of the two test points on the fuse. If it lights up, that side has power. Now test the other point. If both sides light up, the fuse is good. If only one side lights up, the fuse is blown, because power is reaching the fuse but can’t pass through to the other side. This method lets you check an entire fuse box in a minute or two without removing a single fuse.
Cartridge and Thermal Fuses
Not all fuses are see-through. Cartridge fuses, commonly found in older homes, appliances, and some vehicles, have opaque ceramic or plastic housings. You can’t inspect the filament visually, so a multimeter test is the reliable method. Look for external clues too: a cartridge fuse that’s discolored, cracked, warm to the touch, or smells burnt has likely failed.
Thermal fuses are a different type entirely. Found inside dryers, microwaves, and other heat-producing appliances, they’re designed to blow permanently if the appliance overheats. The most common sign of a blown thermal fuse in a dryer is that the machine won’t start at all. In some models, the drum will spin but produce no heat, because the fuse has cut off the heating element. Thermal fuses don’t reset. They look like small plastic or metal cylinders wired inline, and the only way to confirm failure is a continuity test with a multimeter. If there’s no beep, replace the fuse.
Identifying the Right Replacement
Automotive blade fuses follow a standardized color code tied to their amperage rating. Some of the most common colors you’ll encounter:
- Tan: 5 amps
- Brown: 7.5 amps
- Red: 10 amps
- Blue: 15 amps
- Yellow: 20 amps
- Clear: 25 amps
- Green: 30 amps
The amperage is also printed on the top of the fuse in small numbers. Always match the replacement to the same amperage as the original. Your vehicle’s owner manual or the inside of the fuse box cover will show a diagram listing which fuse protects which circuit and what rating it needs.
Why You Should Never Use a Higher-Rated Fuse
It can be tempting to swap in a higher-amperage fuse when the correct one keeps blowing, but this is genuinely dangerous. A fuse is designed to be the weakest link in a circuit on purpose. It sacrifices itself so the wiring and components behind it don’t overheat. A higher-rated fuse lets more current flow than the wires were built to handle. The wires heat up, insulation melts, and the risk of fire climbs sharply.
Incorrect fuse ratings and improper wiring are among the most common causes of accidental electrical fires in the United States, according to the U.S. Consumer Product Safety Commission. A circuit designed for 15 amps that receives 20 because of an oversized fuse will stress every component downstream: capacitors, resistors, switches, and the wiring itself. Over time, this leads to premature equipment failure or worse. Some appliance manufacturers will void your warranty if they find a mismatched fuse was used. If a fuse keeps blowing, the fuse isn’t the problem. Something else in the circuit is drawing too much current or has a short, and that’s what needs to be found and fixed.
What a Repeatedly Blown Fuse Means
A single blown fuse is usually nothing to worry about. A brief power surge or a momentary overload can pop one. But if the same fuse blows again shortly after you replace it, there’s an underlying issue. In a car, common culprits include a short circuit in the wiring, a failing motor (like a window motor or blower fan drawing too much current), or water getting into an electrical connector. In a home, it often points to an overloaded circuit with too many devices or a faulty appliance.
Before replacing a fuse that’s blown a second time, unplug or disconnect the devices on that circuit one at a time. Replace the fuse and see if it holds. When the fuse blows again after reconnecting a specific device, you’ve found your problem. If the fuse blows with everything disconnected, the issue is in the wiring itself, which typically needs professional attention.