You can check if a fuse is blown in two ways: a quick visual inspection or a definitive test with a multimeter. Visual checks work well for glass tube fuses, where you can see the internal wire, but a multimeter gives you a clear yes-or-no answer for any fuse type in seconds.
Start With a Visual Inspection
If you’re dealing with a glass tube fuse, hold it up to the light. Inside the glass, you’ll see a thin wire running between the two metal end caps. In a good fuse, that wire is intact. In a blown fuse, the wire will be visibly broken, with a gap somewhere along its length. You may also see a dark, cloudy residue coating the inside of the glass, which is vaporized metal from the wire melting during a surge.
Automotive blade fuses (the small, flat, colorful ones in your car’s fuse box) have a tiny visible element between two prongs. Look at the fuse from the top or hold it to the light. If the thin strip connecting the two sides is broken or has a gap, the fuse is blown. Some blade fuses are opaque, though, which makes a visual check unreliable.
Ceramic fuses, common in older appliances and some European electrical systems, give you nothing to see. The ceramic housing is completely opaque. For these, you need a multimeter.
How to Test a Fuse With a Multimeter
A multimeter removes all guesswork. This works on every fuse type: glass, ceramic, blade, or cartridge. You have two options for the test, continuity mode or resistance mode, and both tell you the same thing.
Continuity Mode
Turn your multimeter dial to the continuity setting, usually marked with a small speaker or sound wave icon. On many models, this shares a position with the resistance (Ω) setting, and you may need to press a button to switch between them. Plug the black lead into the COM jack and the red lead into the jack marked with Ω or V.
Touch one probe to each metal end of the fuse. It doesn’t matter which probe goes on which end. If the fuse is good, the multimeter will beep, telling you there’s a complete path through the fuse. If the fuse is blown, you’ll get silence and the display will show “OL” (open loop), meaning the circuit is broken and no current can pass through.
Resistance Mode
If your multimeter doesn’t have a continuity beep, switch to the lowest resistance (Ω) range instead. Touch the probes to each end of the fuse. A good fuse reads 0 ohms or very close to it, meaning electricity flows through with almost no resistance. A blown fuse reads “OL” or “1” on a digital display, indicating infinite resistance. The wire inside has melted and there’s no longer a path for current.
Finding Your Fuse Box
Before you can test anything, you need to find the right fuse. In a car, there are typically two fuse boxes: one inside the cabin and one in the engine bay. The interior box is almost always on the driver’s side, either under the dashboard or behind a panel near the steering wheel. The engine bay box sits near the battery under a black plastic cover. Some vehicles have a third location. Ford F-150s, for example, have a junction box behind the glove compartment. Volkswagen Tiguans tuck an extra fuse box under the driver’s seat.
Your owner’s manual has a fuse map showing which fuse controls which circuit. The inside of the fuse box cover often has this diagram printed on it too. If your radio stopped working, for example, the map tells you exactly which slot to pull.
In a house, the main electrical panel is usually in the basement, garage, or utility room. Older homes built before the 1960s may still use fuses instead of circuit breakers. You’ll know the difference immediately: fuses screw in or slot into holders and must be replaced when they blow, while circuit breakers are switches that flip to a middle or “off” position and can simply be reset.
Understanding Automotive Fuse Colors
Automotive blade fuses follow a standardized color code tied to their amperage rating. This matters because replacing a blown fuse with the wrong amperage can either leave the circuit unprotected or cause it to blow again immediately. The most common ratings 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 replace a blown fuse with one of the same amperage and physical size.
What a Blown Fuse Tells You
A fuse blows because more current flowed through the circuit than it was designed to handle. Fuses respond extremely fast to surges, faster than circuit breakers, which is why they’re still used in cars and sensitive electronics. The fuse sacrifices itself to protect the wiring and whatever device is on that circuit.
If you replace a blown fuse and it blows again right away, that’s a sign of a deeper problem. Something on that circuit is drawing too much current, either a short circuit in the wiring or a failing component. A single blown fuse after years of normal use is usually just a one-off surge. Repeated blowing on the same circuit points to something that needs professional diagnosis.
Fast-Acting vs. Slow-Blow Fuses
Not all fuses respond to excess current the same way. Fast-acting fuses (sometimes labeled “F” on the casing) blow immediately when current exceeds their rating. They’re the standard choice for most wiring and general electronics.
Slow-blow fuses, also called time-delay fuses (labeled “T” or “SB”), are designed to tolerate brief surges without blowing. Motors, for instance, draw a spike of current when they first start up, then settle to a normal level. A slow-blow fuse rides out that startup spike. These fuses typically have a coiled or thicker element inside rather than a single thin wire.
When replacing a fuse, match both the amperage and the type. Swapping a slow-blow fuse for a fast-acting one on a motor circuit means you’ll keep blowing fuses every time the motor starts. Swapping the other direction, putting a slow-blow where a fast-acting fuse belongs, could leave a sensitive circuit unprotected during a surge.