A process of elimination is used to fix car headlights. It's like a trial and error situation--try one thing to see if it is the cause, and if that's not it, try another and another, until you are able to repair the headlights. First, however, you have to locate the cause of the problem.
Look at the headlight bulb to see if the bulb is blown. It will be discolored or the filament will be broken or melted. If one bulb is blown, it will look different from the other headlight's bulb. If you can't tell any difference by looking, unplug the bulb from behind the lens and take a closer look. To unplug a bulb, twist and then pull it out. It is attached to a plastic harness with several wires extruding from it. Pull it straight out from the plastic holder. If you are still not sure if it is blown after taking the bulb out, take it to an auto parts store and ask them if they can tell if the bulb is blown. If so, replace it.
Locate the car's fuse box. Locate the fuse that completes the circuit that the headlights are on. It should be clearly marked on the fuse box. Look at it to see if it is blown. If you can't tell by looking at it, remove the fuse to see if it is blown. If it is blown, the wire will be melted and you will possibly see marks where heat caused the wire to melt. If it is blown, replace the fuse and test the headlights. If the headlights work and the fuse does not blow again, then you are finished. However, the headlights may work for awhile and then blow the fuse again. A break in the circuit will allow too much current to cross the fuse and it will melt. The cause could be intermittent and hard to locate.
Determine the reason why the fuse keeps blowing. A break in the wire's insulation or sometimes a loose or faulty connection will cause the fuse to blow. Trace the wire from the fuse box to each headlight and back to the other side of the fuse box. Look for places where heat from electrical arcs has occurred or there are breaks in the wire's insulation. Go to each headlight and check the plugs and wiring connecting to the lights. Unplug and wiggle the connections to make sure they are connected properly and are not loose or faulty. Unplugging and re-plugging the connections can sometimes be all that is needed to make the connections secure again. Replace the fuse and test the headlights again.
Use a volt/ohm/amp meter to look for the problem if the fuse blows after completing Step 3. This will require knowing how to use a volt/ohm/amp meter and the location of the wire circuits in the car. Each wire and connection in the circuit will have to be checked to find the break in the circuit. After the break in the circuit is located, a safe repair will need to be made. After each repair is made, replace the fuse and test the headlights.
