Check Fuses

A fuse is designed to protect more important parts of an electrical system from overheating and related damage. When a potentially dangerous current surge occurs, the wire running through the fuse "blows" and cuts the connection to the circuit. This is great for protecting your car or home electrical system, but inconvenient in the short term. Armed with the right tool and a little knowledge, you'll be able to check your fuses quickly when this happens and find out whether you need a replacement.

Steps

Locating the Fuse Box

1. Check your car manual or look in likely places. Many cars have two fuse boxes, and there's no universal standard for where they are placed. You'll save time if you can refer to your car's manual (or find the manual online), look up the location of the fuse that controls the part of the car that lost power, and check it directly. If you don't have the manual, check these common locations for a large box, or a group of exposed fuses:
   • Most cars have one or two fuse boxes under the hood, next to the engine or battery. There may also be a fuse box inside the car, so keep looking if the fuse you're looking for isn't here, or if all the fuses here are fine.
   • Newer cars often have a fuse box underneath the dash in an easily accessible location. Check the ceiling of the glove box for a hinge that swings downward. You may need a flat-head screwdriver to open the lid.
   • Older cars often have their fuses in an open box to the left of the brake pedal or the foot-operated parking brake. It's difficult to examine the fuses closely in some models, so bring a flashlight and/or hand mirror.
   • Less commonly, the fuse box is located in the trunk or under the rear seat.
2. Look for indoor fuse boxes. If you are checking the fuses in your home, look for a fuse box or circuit box in closets, basements, laundry rooms, or on the outside wall of the house. If you live in an apartment or shared home, the fuse box may be located in a neighbor's apartment.
3. Check the user's manual for other devices. If you are checking the fuses in a central air conditioning unit or another appliance, check the user manual to find out where they are. For some appliances, you may need to shut off the power to the device before you can safely access the fuse box.

Checking Visual Signs

1. Read the labels if present. Car fuse boxes often have a diagram on the outside or inside lid, and always in the car's user manual. This can save you a lot of time, since you can check the one fuse that protects the radio (or other nonfunctional system) instead of all 40+ fuses. Home fuse boxes are often unlabeled, but typically only have a few fuses, so it's easy to test them all
   • Find your car's user manual or fuse box diagram online here or with a general online search. You'll need to know the make and model of your car.
2. Leave the fuse connected. Don't remove the fuse yet, as this can be dangerous if the power is still connected, and can cause minor problems to connected systems if you remove a still-functioning fuse. Instead, look closely at each fuse while they are in place.
3. Check for broken wires or scorch marks. A fuse can look completely normal and still be blown (and in need of replacement), but more often than not there will be a visual sign that will guide you to the problem fuse. Fuses come in three basic forms:
   • Transparent cylinders (glass or plastic) have a wire running through the middle. If the wire is broken, the fuse is blown. If the whole cylinder is scorched black or brown, the fuse is blown and there may be a major short on the circuit that needs repair (especially if the replacement fuse blows after a short time).
   • The two-pronged chip fuses often used in cars have a U-shaped wire running between the prongs, inside the plastic. If the wire is broken, the fuse is blown, but it's often difficult to tell.
   • Opaque cylinders (covered by a solid metal coating) must be tested by other means.
4. Turn off power and remove an indoor fuse. If checking an indoor fuse, you can turn off the main power to the house and remove the fuse to get a closer look. For any type of fuse, if you are still unsure, you can move on to the next section. If you have identified the problem fuse, skip ahead to replacing the fuse.
   • This step is not recommended at this stage for car fuses, since removing the wrong fuse can negatively affect idling performance, diagnostic information, or accessories.

Testing the Circuit

With a Test Light

1. Purchase a modern test light. You can purchase this tool at a hardware store or electronics store. Select a "computer safe" model with an LED light in the handle, or with an incandescent bulb powered by a battery or power outlet. Never test car wiring with an old-style test light with an incandescent bulb that draws power from the circuit, or you could trigger the airbags and cause massive damage.
   • Alternatively, if you have a multimeter, skip down to the instructions below to use it instead.
2. Check the fuse with the test light. Follow these steps to check the fuse with a test light:
   • Clamp the black grounding lead to an electrically conductive object (such as anything metal).
   • Turn the ignition to run, or make sure the main power supply is on if testing a house fuse.
   • Touch the red probe near one end of the fuse, then the other. (For small two-pronged fuses, the two prongs are the two ends of the fuse.)
3. Interpret the results. If the fuse is good, the bulb in the test light will light up both times. If it does not light up on one end, the fuse is blown and needs to be replaced.
   • If neither end lights up, there is no current running to the fuse box, or your black lead is not grounded, or the light bulb has burned out. Correct this problem and try again, or use a multimeter instead.

With a Multimeter

1. Turn off the power and remove the fuse. Turn off the ignition to the car, or the power leading to the home fuse box. Pry the fuse out on one end, then the other. You may need tweezers or needle-nose pliers to remove the fuse, or a similar tool sometimes attached to the lid of automobile fuse boxes.
   • If you plan on testing more than one fuse in a car's fuse box, take a photograph first so you know exactly where each fuse goes.
2. Test continuity if available. Most digital multimeters have a continuity setting, which is labeled with a series of parallel arcs: ))). Turn the dial to this setting, then place the two probes at opposite ends of the fuse. If you hear a constant beep while they are touching the fuse, the fuse is still good. If you don't hear anything, the fuse is blown.
   • If your multimeter has no continuity option, or you would like to double-check the result, continue on to test resistance.
3. Set the multimeter to test resistance. This setting looks like the Greek letter omega: Ω. The resistance setting sends a small current through the fuse, and tests how much current goes through. We don't really care about the exact resistance reading, but if the fuse is blown, we won't get any resistance reading because the current will not be able to pass through the broken wire.
   • If you have an analog multimeter, there are multiple Ω settings. Choose the one that says Ωx1. Old models sometimes say Rx1 instead.
4. Touch the two probes to each other. Touch the two metal probes together and look at the multimeter display. This number (or needle position) is what the multimeter currently "thinks" is a resistance of 0. If we get a result near this number when testing the fuse, the fuse is still good.
   • Your multimeter probably has a dial (analog multimeters) or a button (digital) that lets you calibrate this back to 0. You can do this if you plan on using the multimeter a lot, but it's not necessary for this task.
5. Touch the probes to each end of the fuse. Touch one probe to each end of the fuse, while looking at the display. If the display doesn't change when you touch the fuse, the fuse is blown and needs to be replaced. If the number or needle position moves to a similar position to the one you saw when both probes were touching, then the fuse is still good and can be popped back into the fuse box.

Replacing a Fuse

1. Turn off the power and remove the fuse. Whenever you are removing a fuse or putting in a new one, make sure the power supply to the fuse box is off. For a car, this means turning off the ignition.
2. Find a new fuse. These are available at hardware stores, electronic shops, or auto supply stores (for car fuses). Bring along your old, blown fuse so you can compare the size and other characteristics, as described below.
3. Select a new fuse of the same amp rating, type, and shape. Replacing a fuse with the exact same type of fuse is very important. Most importantly, check the amp rating, normally a number written on the fuse, and purchase a replacement fuse with the same rating. Each fuse is designed to blow at a specific number of amps; this is its entire purpose. If you use too low an amp rating, the fuse might blow during normal use and short the circuit. If you use too high an amp rating, the fuse won't blow during a power surge, and the less replaceable parts of the circuit will be damaged instead.
   • Transparent, cylindrical fuses come in two types: fast-blow (a straight wire), or slow-blow (a coiled wire). Don't put in a slow-blow unless the old fuse was also a slow-blow, or the fuse may not be able to prevent damage quickly enough.
4. Pop in the new fuse. Modern fuses should pop in easily with a little pressure. Old, glass fuses sometimes need to have one end pushed in first, then the other.

Tips

• Fuses age just like everything else. Over time, they will fail. A blown fuse is not always an indicator that there is something wrong with the power circuit.
• If a blown fuse is preventing your car from working, but you need to drive to pick up a new fuse, consult your car's manual and remove a fuse with the same ampere rating from a nonessential system, such as the radio. Put it in place of the broken fuse.
• If the replacement fuse blows shortly after installation, and you're sure you used the correct amp rating, there could be a more serious issue with your electrical system. Consult an electrician.

Warnings

• Make sure that the power is turned off to the equipment before testing any fuse or removing any fuse, unless you are using a test light.

Things You'll Need

• Multimeter

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