At some point or another, we've all had some electrical device or system quit working on our vehicle. While electrical systems on modern cars can fail for any number of reasons, the first thing you should do when something quits working is check the fuse.
Finding the Fuses
When checking fuses, the first thing you have to do is find them. Keep in mind that many vehicles have more than one fuse box, and common locations for them can include under the hood, under or on the side of the dashboard, behind the glove box, underneath the rear seat, or in the trunk.
Fuse boxes will have a plastic cover over them, and the other side of the cover will have a diagram that will indicate what circuit (or circuits) each fuse is for.
If you're having a hard time finding the fuse you need, your owner's manual will list the locations of all fuse boxes and will likely also have a diagram of what each fuse is used for.
A good fuse (left) compared to a blown one (right). Note how the center strip is burned up in the bad fuse.
While there is some variation when it comes to fuse styles, most vehicles use blade type fuses. Some modern vehicles will also use cartridge fuses, while a number of older vehicles (such as those built before 1980) often use glass fuses.
Fuse Sizes and Ratings
Automotive blade fuses come in a number of sizes, but the most common ones seen are mini, standard, and maxi. Regardless of size, current ratings (in amps) will be printed on the fuse and they are also color coded. Cartridge fuses come in a few different styles, but will also have current ratings printed on them. However, the colors and ratings of cartridge fuses will not match those of blade fuses. Glass fuses with different amperage ratings vary in length, which helps prevent the use of an incorrectly rated fuse.
How to Check Fuses by Hand
While finding the fuse you need to check may require a little bit of legwork, actually checking a fuse once you've found it is quite easy.
Locate the fuse box and remove the cover.
Using the diagram on the back of the fuse box cover as a guide, find the fuse for the circuit or device of interest (the taillight fuse, for example).
Using a fuse puller or a small pair of needle nose pliers, remove the fuse and check its condition (many vehicle fuse boxes will have a fuse puller inside).
Regardless of type, if the fuse is blown then the metal strip in the center will be melted or disconnected. The fuse most likely to blow is the one for the cigarette lighter or auxiliary power outlet in the vehicle. Having too many devices or ones that draw too much current from the cigarette lighter can cause the fuse to blow, as can conductive objects (such as pennies) falling into the cigarette lighter and shorting it out.
How to Check Fuses with a Test Light
If you are handy and have a test light, you can also use that to quickly check a fuse without having to pull it.
Clip the test light lead to a piece of unpainted metal on the vehicle (such as on the alternator bracket).
Turn the ignition key on, but leave the engine off.
One at a time, touch the end of the test light on each of the two metal terminals on the top of the fuse.
If the fuse is good, the light will come on when both terminals are touched. If the test light only lights on one of the terminals but not the other, then the fuse is blown.
Replacing a Fuse
A vehicle’s fuse box will often have a few spare fuses inside, so check the diagram on the cover for their location. If not, fuses are inexpensive and can be purchased at most stores. While checking fuses is pretty easy, replacing them is even easier:
Put a new fuse in place of the old one (you may have to give it a firm push to seat it fully).
Verify that the circuit or device that had the blown fuse is now functioning.
Put the fuse box cover back on.
When replacing fuses, be sure that the replacement fuse has the same amperage rating as the one you took out.
If you replace a fuse with one that is rated too low, it will likely blow prematurely.
If you replace a fuse with one that is rated too high, there is a chance of vehicle damage or fire.
Never use a paperclip, penny, or any other metal object to bypass a fuse as this can lead to vehicle damage or a fire.
If you are in a bind and don’t have a replacement fuse of the same rating available, you can use a fuse that is next lower rating only to get by until you can get the correct replacement fuse. For example, if the fuse that blew is a 20 amp fuse and you don’t have another 20 amp to replace it with, you can use a 15 amp fuse to get by until you get the right replacement.
You can purchase aftermarket fuses that have a small LED light built in. If the fuse is intact this light will be off, but if the fuse blows then the light will illuminate. These known as lighted or Smart Glow fuses, and they make it extremely easy to identify a blown fuse. While convenient, keep in mind that they do cost a little more than regular fuses.
What If the Fuse Just Keeps Blowing?
If the replacement fuse blows again or you have to replace the same fuse often, have the vehicle serviced. There is likely more going on than just a one-time current spike and the circuit or device at hand may need to be repaired by a professional.
Get Your Vehicle Serviced if You Notice:
A fuse that blows repeatedly
A circuit or device doesn’t work when the fuse is good
Rust or corrosion at or near the fuse box
Burned or melted wires anywhere in the vehicle