Today’s car headlights are long-lived, very reliable, and throw more light onto dark roads than older-style lights. But even such low-maintenance items need your attention occasionally. One of the two most common malfunctions may not always be easy to spot: misaligned lights. The other could hardly be missed: failure to shine. In many cases, either condition can be corrected with a minimum of tools, skill, or money. It usually takes only one driver to change a headlight.

Cars have either sealed beam headlights or replaceable bulbs. Some years ago, car makers started switching over to the bulb type, but there are plenty of sealed beam cars still on the road. Sealed beam lights are the big, glass, bulb-like units with the silvered reflector built in. Newer sealed beam units are halogen lights, which shine whiter and brighter than older, conventional sealed beams.

Whichever kind your car has, the test for alignment is so satisfyingly low tech that it strains nobody’s mechanical talent. To be sure your headlights point their beams straight ahead and parallel with the road—and not in the eyes of approaching drivers or off into space—park the car on level ground facing a wall about 25 feet away.

When it gets dim enough, turn the headlights on and verify that the beams are both parallel to the ground and striking the wall directly in front of the car. This method isn’t precise, but will let you know when the headlights are significantly askew. In many cases you can correct the alignment by adjusting the aiming screws. There are two per headlight, directly by it. If you’d rather not do it yourself, the lights can be professionally aimed with greater precision for about $25-$40.

When a headlight fails and the rest of the electrical system works, the culprit is almost always the bulb or sealed beam. In many cases, either can be replaced by even the mechanically challenged.

To get the correct bulb or sealed beam, know the make, model, and year of your car when you go to the auto parts store. And, if your vehicle has four headlights, remember which one you’re replacing. If the car has sealed beams, you can replace both the headlights with the brighter halogen beams, whether they’re original equipment or not. These are recognizable by the word "Halogen" written on the lens.

Sealed beams. Getting to the screws that hold the light-retaining hardware in place can be a mild adventure, as you might have to disassemble a layer or two of trim and what’s holding that in place isn’t always obvious. In any case, be sure the engine and lights are off before starting.

Once you’ve reached the sealed beam, remove the retaining screws. Gently pull the unit out, reaching around behind it with your spare hand, grasping the socket, and detaching it.

Plug the new unit in and, before reassembling everything, turn the light on to make sure it works. Put the retaining hardware back in place and check the light’s aim. In most cases it will be OK, but adjust as needed then reinstall anything else you had to take off the car to get at the light.

Bulbs. Most newer cars have halogen bulbs instead of sealed beams. Bulbs are smaller than sealed beams and not built into a single unit with the reflector. Because they can be difficult to get at, they are probably the most challenging light bulbs you’d ever have occasion to change.

Unlike sealed beams, bulbs are approached from the rear. Open the hood and search in the general area behind the headlight. So much equipment is crammed into the limited space beneath most car hoods these days that getting to the bulb can try the patience. Persevere.

Behind the bulb you’ll find a retaining ring. Unscrew it, then pull both socket and bulb directly backward. Disconnect the bulb from its socket. It may be necessary to wiggle the bulb around a bit to loosen it from the socket, but remember that auto electronics usually respond spitefully to force.

Plug in the new bulb, test it, then put everything back the way you found it. When handling halogen bulbs, it’s important not to touch them with your bare hands as skin oils weaken the crystal and can cause early bulb failure. Use a paper towel or clean cloth.

Although most cars have lights that are relatively straightforward, even changing a bulb can become a federal case with a few cars. Some vehicles, such as certain Acuras and BMWs, have projector beam Xenon bulbs; these can be impressively expensive and potentially troublesome to replace; they’re best left to those who know what they’re doing.

• Whatever kind of headlight you have, clean the lenses when you get gasoline. This is especially worth remembering if your car has the kind of lights that pop up when you turn them on. They’re almost never handy when you’re at the gas pump, so can go uncleaned until they collect a truly lumen-reducing burden of dirt.