How to test and replace car fuses

When an electric accessory or a light (except for headlamps) fails to work, the cause may be a blown fuse. Fuses blow when there is too much current flow. However, nonrecurring conditions can also cause a fuse to blow. If a new fuse blows almost immediately, there is a circuit problem.

The fuse box may be under the dash, under the hood, or in the glove box (check your owner’s manual). The fuse box is marked so that you can find the fuse for the affected circuit.

There are three types of car fuses: glass envelope, ceramic body, and minispade. When one blows, its metal strip breaks (you may not be able to see the break with the fuse in the holder). To check, use a 12-volt test lamp with a pointed probe.

Connect the lamp’s alligator clip to body metal, turn on the problem circuit, and probe each end of the holder of a glass or ceramic fuse. With a minispade fuse, probe through the holes in the ends of the fuse. The test lamp should light in each case. If it lights in only one, the fuse is blown; if it lights in neither, there is a wiring problem.

Turn off the ignition and affected circuit switches. Use a special puller to remove a glass-envelope fuse. Remove a minispade or a ceramic type with your fingers.

Install a new fuse of the same amperage as that stamped on the fuse. A higher-rated fuse won’t protect the circuit. A lower-rated one will blow even if there is normal current flow.