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What Does a Blown 40 Amp Fuse Look Like? (w/ Pictures)

Written by Edwin Jones / Fact checked by Andrew Wright

what does a blown 40 amp fuse look like

Fuses, unlike circuit breakers, need to be replaced. In most cases, you need to rely on visual clues to know this for sure. So what does a blown 40 amp fuse look like?

Even though fuses have different types and looks, whether they’re automotive- or appliance-based, they share similar tell-tale signs.

These include a split in the visible internal strip/line and a cloudier, typically charry look, especially in the portion where the disconnect occurred. The fuse’ amperage doesn’t play much of a role in the diagnosis.

This answer applies to two-prong blade fuses and glass tube varieties.

Primary Signs of a Blown 40 Amp Fuse


Normally, a fuse blows fast, and you’re only left with the ensuing aftermath, which has fairly obvious signs.

  • Notice how the metal strip connecting the circuit immediately breaks, with parts of it becoming molten.
  • That’s literally how hot it can get inside the casing, so the blackened, smoky aftereffect is a pretty normal occurrence, too.

That’s essentially how to know if car fuse is blown, regardless of whether it’s a 40 amp fuse blown, 13a fuse, or a 50 amp fuse. A 40 amp slow blow fuse will disconnect the circuit in the same way; it’s just that it’s able to sustain loads that exceed its rating longer.

How about for glass cartridges? What if the fuse is blown in-house, where glass cartridge and screw-in fuses are normally used? In such cases, the same clues apply.

There may not be as much blackening on a cartridge fuse at times, but it will often be present to certain degrees.

Keep these signs in mind when trying to figure out what burnt fuse look like based on other variations:

  • In a screw-in house fuse, the window may look fully black and burnt, with the copper strip virtually invisible as a result.
  • A Jcase fuse will also have an alloy line, which will also look visibly cut, but there may be less discoloration.

Overall, what a broken fuse looks like may mirror the aftermath of a lightning strike or a clean cut on a metal strip. If you get the former a lot with a bad 40 amp fuse, you shouldn’t be surprised because, as the video, I shared above states, it’s often the result of excessive current flowing through it.

Incidentally, I don’t get why people say things like “relay fuse is blown without a multimeter” online. A relay and a fuse are two different things; the only function they share is that they’re both intended to interrupt the circuit during certain conditions.

What is the 40 Amp Fuse Used for in Cars?

Blade fuses and Jcase fuses normally serve cars, unlike the screw-in and cartridge varieties used in homes. They both fulfill the same purpose of protecting a particular component of vehicles from the damaging effects of overloads, short circuits, and arc faults.

A 40A fuse ideally serves a component that has that exact or nearly the same current draw. Such high-draw parts may be the CDI ignition, audio amplifiers, or electric fans.

How to Replace a Blown 40 Amp Fuse?


Before buying a replacement 40 amp fuse AutoZone or other retailers offer, it’s important to pinpoint and solve the issue that caused it to blow.

For example, I’ve had a client approach me wondering why her 40 amp ignition fuse keeps blowing, and it turned out to be an issue with the relay, which had to be replaced.

In short, blown fuses don’t necessarily mean there’s a problem with your appliance or car. Feel free to consult a mechanic to know for sure.

Once you’ve remedied the problem behind your burnt fuse in your car, then you can start following these steps:

  • Locate your car’s fuse box. To be sure and to save time, refer to your car’s manual. The fuse box is usually under the dashboard, though you can look near your seat or in the glove compartment as well.
  • Some cars show information about which fuse serves a specific circuit behind the box’s cover. Pinpoint the one that’s connected to the component you’re having issues with. Alternatively, refer again to the manual.
  • Use a fuse extractor to pull the blown fuse out. Once done, you only need to position the new fuse in place using the extractor, then push it down with your finger.



How do you test a 40 amp fuse?

This is a great skill to have, especially if you intend to replace all your fuses by yourself. In most cases, a test light will do, especially if it’s a car fuse.

Assuming you already have the fuse, new or otherwise, you just need to place the tip of the test light on the fuse terminals you wish to test. The light should illuminate, because if not, then the fuse has likely blown.

For a home fuse, you can use a multimeter. Set it to the resistance measurement setting. Afterward, touch the two pins on both ends of the cartridge fuse. It should display the same value as when you touch the two pins together, and not ‘OL’, depending on the multimeter you’re using.

What happens when a 40 amp fuse blows?

It will protect the component and the circuit it’s connected to from the possible damage that short circuits, arc faults, and overloads may inflict.

If the fuse happens to be for lights, they may not turn on. Most will not work altogether or may only manage limited functionalities.

How much is a 40 amp fuse?

Fuses are usually sold in sets. You can buy a 40A single Jcase fuse at Walmart for less than $5, while a set of 4 two-prong blade fuses sell for $6.


If you happen to have a buddy who asks you, “What does a blown 40 amp fuse look like?” you should now be able to provide the answer using the points I’ve shared here.

Remember: you can immediately tell a blown car fuse apart from a normally functioning one by juxtaposing the two. Always, a bad fuse look like it has seen better days, considering the glaring cut in the metallic strip and the scorch marks that often stick to the casing and windows.

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