How to Check a Fuse in a Car

Fuses are safety devices that protect electric circuits in a car from excessive current (overloading). Without a fuse an overloaded wire could melt or catch fire. Each circuit is protected by its own fuse. Several larger fuses with high Amp rating protect multiple or high-current circuits, such as electric power steering or radiator fan circuits. There is also at least one main fuse. The fuse removal tool is also typically located in the fuse box or in the fuse box cover.

When any electrical component in a car doesn't work, the first step is to check the fuse that protects the circuit with this component. Modern cars typically have two fuse boxes: one under the hood, and the other inside the cabin. You can find the fuse map in the owner's manual, or on the fuse box cover. There are several ways to check fuses. The easiest way is to pull the fuse out and check it visually; you can find the directions in the vehicle's owner's manual.

How to check the fuse visually

For example, in this car, the cigarette lighter socket, also known as the front accessory power socket, stopped working. We are going to check the fuse. In this car, the inside fuse panel is installed right above the driver's kick panel. We found a tool to pull fuses inside the fuse box in the engine compartment.

Turn the key to the OFF position. Before pulling the fuse, It's always a good idea to mark its position, so you can install it into the same spot. We take the tool and pull the fuse with it. As you can see, the fuse is blown (on the right). Fuses have a thin metal conductor inside that melts when the current is higher than the fuse rating. In this blown fuse the conductor is melted.

If a fuse is blown, something has shorted the protected circuit. If the problem is not fixed, the fuse will blow again. In this car, it was a small screw that fell into the front cigarette lighter socket. Spare fuses in this car are also located inside the fuse box cover in the fuse box under the hood. When replacing a fuse, use only the proper fuse type. This one is a 15-Amp fuse; in most cars, it comes blue. Some types of fuses, like these low-profile mini fuses in the photo above, are universal and can be purchased in any auto parts store. Larger fuses or panels with several fuses might need to be ordered from a dealer.

How to check the main fuse

All cars have at least one main fuse or fusible link. It's usually installed at the positive battery terminal or in the fuse box, connected to the battery positive cable. Often the main fuse blows when accidentally touching the wrong battery terminal when boosting a dead battery. Checking the main fuse is easy, it's usually clearly visible if it's blown. If the main fuse is blown, there is a chance that a few other smaller fuses are blown too.

How to check a fuse with a multimeter

If you have a multimeter, there are two ways to check a fuse. The first way is to measure the voltage at both pins (blades) of the fuse. Small fuses in the car have the top portion of both pins protruding through the top of the fuse, see the photo. This allows measuring the voltage at each side of the fuse without pulling it out.

Set the multimeter to DC (Direct Current). Connect the COM (black) terminal to the negative battery terminal or a metal part that is connected to the vehicle chassis or body. Set the parking brake and turn the ignition to the ON position. The ignition needs to be ON, because with ignition OFF, not all fuses are powered. With the positive probe, check the voltage at both sides of each fuse. A fuse is just an electrical conductor. If both sides show 12 Volts, the fuse is good.

If one side shows 12 Volts, white the other shows no voltage, you have found the blown fuse. This method works well, when many fuses need to be checked at once. Some mechanics use a power probe instead of a multimeter. A power probe lights up when there is 12-Volt power.

How to check fuse resistance with a multimeter

If you already pulled the fuse out, but it's not clear if it's blown or not, you can check its resistance. The resistance is the opposite of the current flow. The lower the resistance, the higher the current flow. The resistance is measured in Ohms or Ω. A conductor, like a cooper or aluminum wire, has very low resistance (close to 0 Ω). A good fuse will show 0 (or close to 0) Ohm, see the right image. In other words, there is a continuity between two pins (blades) of the fuse. A blown fuse will show very high resistance (infinity), see the left image.

To measure resistance of any electric component, it has to be disconnected from the electric circuit. You can't measure electrical resistance while the component is plugged in or powered. Switch the multimeter to Ohms and connect the probes as in the photo.

What can cause a fuse to blow?

A fuse protects the circuit from higher current than the circuit can handle. If a fuse is blown, it means there is a short cirquit somewhere, whether between two wires or the power wire and the ground (car body). A fuse can also blow if a component draws a higher current than it's designed for. For example, if a wiper motor or a blower motor is jammed when powered on, it will draw higher electrical current and possibly pop the fuse. The same can happen if a winding has shorted inside the motor. We noticed a few common problems that cause blown fuses in many cars:
1. The most common case is when a metal object (e.g. coin) falls inside or a shorted accessory is plugged into the front accessory power socket (cigarette lighter socket).
2. The wire harness that goes into the trunk lid or liftgate breaks in the place where it bends, shorts and pops the fuse related to taillights or brake lights.
3. The wrong bulb installed in one of the headlights or taillights can also pop the fuse.
4. The damaged or rubbed through trailer harness shorts out.
5. A wiring harness that is connected to some component inside the engine bay rubs through and shorts the fuse. In some older Mercedes-Benz cars, for example, it was a fairly common problem when the insulation on the engine wiring harness cracks shorting the wires and popping the fuses.