These days, nearly every component of a vehicle is electrically powered, from the interior lights and seat controls to the anti-lock brakes and engine control unit. Fuses are put in place to protect the wiring and electrical circuits from burning and shorting out, respectively. These tiny contraptions typically have a plastic encasement around a conductor and two prongs, or blades, that fit into sockets. The conductor is made of metal that has a lower melting point than the wire it protects.
If there is too much electricity flowing through the circuit, the conductor will melt. This breaks the circuit and prevents the wire from overheating. Blown fuses are easy to replace - just remove the old one and insert a new one in its place. Often, you will be able to remove a fuse by hand, but some may require the help of tweezers. Wires, on the other hand, are far trickier to swap out when they are damaged.
Fuses come in a variety of amperage, sizes and designs to handle the varying currents needed for different electronic devices. A cooling fan, for example, will likely need more electricity to power it than an interior light.
Most vehicles have two separate fuse panels. The fuse panel under the hood is designated for the engine components, and the fuses located under the dash typically correlate to electrical devices in the passenger cab. Often, there will be a guide posted near the panel to explain which fuse is responsible for which car part, and the owner's manual should also contain this information.
The tried-and-true method for determining whether a fuse has blown is to physically remove it and visually inspect it. The plastic casing of a fuse is translucent, so you can easily see the conductor within. If the metal filament is broken, then the fuse will need to be replaced. In older vehicles, the fuses may be glass tubes, but the method of visually determining if they have blown remains the same.
It is extremely important that a new fuse has the same current rating as the one it will replace. There are four different size groups for automotive fuses - from smallest to largest, they are low-profile mini, mini, regular and maxi. The blades of low-profile mini fuses, also called micro fuses, are build into the plastic housing, while the blades of the other sizes stick out like prongs from the bottom of the fuse.
Fuses also come in a wide variety of colors, and each color denotes a different amperage, which is also marked by a number on top of the fuse's plastic case. This makes it easier to find the correct replacement. A fuse with the incorrect amperage could damage the wiring or cause a short circuit. A fuse with too low of a current rating is likely to fail quickly and you'll need to replace it again, while too high of an amperage will allow too much electricity to flow, and can damage the component it is connected to.
Unless an electrical device is acting up or not working, the fuses can generally be left alone. However, if after you replace a blown fuse it burns out again, you may want to take a closer look at the system it is attached to. This could point to an issue with the wiring or other aspects of the particular device.