A blown fuse usually gives itself away in one of two ways: a visible break in the metal strip inside the fuse, or a device that suddenly stops working. In many cases you can confirm it with nothing more than your eyes, though a simple multimeter test removes all guesswork. Here’s how to check every common type of fuse you’re likely to encounter.
Visual Signs of a Blown Fuse
Most fuses are designed so you can see the thin metal filament inside. When that filament melts from excess current, it leaves behind obvious evidence. What you’re looking for depends on the type of fuse.
Blade fuses (cars and trucks): These are the small, colorful plastic fuses in your vehicle’s fuse box. Pull the fuse out and hold it up to a light. A good fuse has an unbroken metal strip connecting the two prongs. A blown one will have a gap in that strip, often with melting visible around the break. Brown or black burn marks inside or on the outside of the plastic housing are another giveaway.
Glass tube fuses: Common in older vehicles, audio equipment, and some household items, these let you see the filament clearly through the glass. A broken wire inside is the most obvious sign. If the inside of the glass looks blackened or has a metallic residue coating it, the fuse overloaded and is dead.
Ceramic cartridge fuses: These are opaque, so visual inspection won’t help. You’ll need to test them with a tool (more on that below).
Symptoms That Suggest a Blown Fuse
Sometimes the fuse itself isn’t easy to reach or inspect, but the device it protects tells you what happened. A single circuit going dead while everything else works normally is the classic pattern. In a car, that might mean your radio cuts out but the headlights still work, or your power windows stop responding on one side. In a house, one outlet or group of outlets goes dark while the rest of the room has power.
Appliances have their own version of this. A dryer that won’t start at all, or one that tumbles but produces no heat, often has a blown thermal fuse. Microwaves that are completely unresponsive despite being plugged in can have the same issue. The common thread is a sudden, complete loss of function in one specific device or circuit, with no gradual decline beforehand.
Testing With a Multimeter
A multimeter gives you a definitive answer, especially for fuses you can’t see through. You have two options: continuity mode or resistance mode. Both work equally well.
Continuity Test
Turn your multimeter’s dial to the continuity setting, which is usually marked with a small diode symbol or the word “CONT.” Touch one probe to each end of the fuse (or each metal prong on a blade fuse). If the meter beeps, the fuse is good. If there’s silence, the fuse is blown. Some digital multimeters also display a number during this test. A very low reading means the fuse is intact. A reading of “OL” (overload) or a very high number confirms the fuse has failed.
Resistance Test
Set the dial to the resistance setting, marked with the Greek omega symbol (Ω). Touch the probes to each end of the fuse. A good fuse reads near zero ohms, meaning electricity flows through it with almost no resistance. A blown fuse shows “OL” or infinite resistance, meaning the circuit is completely broken inside.
Testing Car Fuses With a Test Light
If you don’t own a multimeter, a basic automotive test light works for car fuses, and you don’t even need to pull the fuse out. Clip the test light’s alligator clamp to the negative battery terminal. Then touch the probe tip to one of the small exposed metal tabs on top of the fuse, and then the other tab.
If both sides light up, the fuse is good. If one side lights up and the other doesn’t, the fuse is blown. If neither side lights up, turn the ignition key to the accessory or “ON” position and try again. Many car circuits only receive power when the key is on, so a fuse can appear dead simply because its circuit isn’t active yet.
Indicator Fuses That Tell You Themselves
Some newer blade fuses have a built-in LED that lights up when the fuse blows. Littelfuse makes a popular version called the Smart Glow series. These look and fit just like standard blade fuses, but they contain a tiny LED circuit that activates only when the main filament breaks. If you swap your vehicle’s standard fuses for indicator versions, diagnosing a blown fuse becomes as simple as scanning the fuse box for a glowing light. The fuses are color-coded to the same amperage standards as regular blade fuses, so they’re direct replacements.
Fuse Color Codes for Correct Replacement
Once you’ve confirmed a blown fuse, replacing it with the wrong amperage rating is dangerous. Automotive blade fuses follow a universal color code that makes this straightforward:
- Tan: 5 amps
- Brown: 7.5 amps
- Red: 10 amps
- Blue: 15 amps
- Yellow: 20 amps
- Clear: 25 amps
- Green: 30 amps
- Orange: 40 amps
The amperage is also printed on the top of each fuse. Always replace a blown fuse with one of the same rating and type. Using a higher-rated fuse won’t protect the circuit properly and can cause wiring to overheat. Using foil, wire, or any other improvised substitute creates a serious fire and electrocution risk.
Safety Before You Start
For household appliances, unplug the device from the wall before removing or inspecting any fuse. For equipment hardwired into your home’s electrical system, switch off the relevant breaker first. Skipping this step with household voltage can cause serious injury.
Car fuses are lower voltage and generally safer to handle, but turning the ignition off before pulling fuses is still good practice, particularly around higher-amperage circuits.
If a new fuse blows immediately after you install it, stop replacing it. A fuse that keeps blowing points to a short circuit or an overloaded wire somewhere downstream. That’s a problem for a qualified electrician or automotive technician to trace, not something a fresh fuse will fix.