headlight FAQs
Home, Auto Repair Library, Auto Parts, Accessories, Tools, Manuals & Books, Car BLOG, Links, Index


Headlights: Frequently Asked Questions

by Larry Carley copyright AA1Car.com

Q. How long do headlights last?

The life of the average halogen headlight is about 4 to 6 years. It depends how much night driving you do and how many hours the headlights are used (day or night). HID headlight bulbs do not have filaments so they are much longer lived (10 years or more) while LED headlights have no filament or glass bulb and will last tens of thousands of hours, which is usually the life of the vehicle.

Q. What causes headlights to fail?

The filament inside an incandescent bulb or halogen bulb is the weak link. It operates at extremely high temperature, and is brittle. Over time, the filament will thin and eventually break. Excessive vibration (like driving on extremely rough roads) may also cause the filament to fail prematurely. Rocks kicked up by traffic can also break out a headlight if it penetrates the outer cover and hits the bulb inside. Electrical faults in the headlight wiring, connectors, relays or lighting control module may also prevent a headlight from working.

Click Here for more help troubleshooting headlight problems.

headlights dim

Q. Do headlights dim over time?

Yes, but the amount of dimming of a halogen headlight is only about 10 percent over the bulb's life. Tungsten particles that evaporate from the hot filament will gradually form a dull gray or blacking coating on the inside of the bulb over time, which reduces light transmission through the glass and light output. The light output of LED headlights can also dim over time, but because they are so long lived most LED bulbs will maintain their brightness over the life of the vehicle.

faded plastic headlight cover

Sun faded plastic headlight covers can also reduce light output. Plastic covers made of Lexan are crystal clear when new, but can discolor, fade and reduce light transmission after years of sun exposure. The UV in sunlight reacts with the plastic to form a milky or dull film on the surface of the headlight covers. Buffing the plastic covers with a mild abrasive polish or rubbing compound can remove this dull film and restore both a like-new appearance and the brightness of the headlights.

Q. How much should it cost to replace a headlight?

Standard halogen bulbs in most auto parts stores retail for around $12 to $17 each (less if you buy a two pack). High performance halogen bulbs that produce more light can cost $15 to $23 each (less if you buy a two pack). HID bulbs are much more expensive costing from $100 to $150 each! A sealed beam headlight for an older vehicle may cost $15 to $25 depending on the application.

Q. Should you replace both headlights at the same time if one has burned out?

Yes. Both headlights have the same number of hours of use, so if one headlight has burned out the other headlight is probably near the end of its life too.

Q. How do you replace a headlight?

Halogen bulbs and HID bulbs are usually inserted into the back of the headlight housing from inside the engine compartment. Many bulbs are a twist-lock design. You twist the bulb connector counterclockwise to unlock the bulb from its housing, then pull out the connector and change the bulb.

Some bulbs are held in the back of the headlight housing by metal clips, which must be carefully removed to change the bulb.

On some vehicles, access to the back of the headlight housing may be restricted or blocked by other components that are in the way, which mean you may have to remove some covers, trim or the headlight housing itself to change the bulb.

When installing a new bulb, DO NOT touch the glass with our bare fingers because the oils on your skin can contaminate the glass and cause it to crack when the bulb gets hot. Handle the bulb by the plastic base only.

Q. Are headlight bulb upgrades worth the money?

It depends what you want. If you do a lot of night driving and are tired or replacing bulbs more frequently than usual, buy a pair of long life bulbs that provide 2X to 3X the lifespan of a standard bulb.

If you want better nighttime visibility, there are bulbs that offer a brighter, whiter light output. However, the trade-off for more life is often shorter bulb life. Refer to the packaging on the product to see what the manufacturer says bulb life and light output.

aftermarket custom headlights

Q. Are colored headlights street legal?

It depends on the color of the lights and local regulations. In most cities and states, the only headlights that are street legal are white, yellow (fog lamps). or sometimes blue (some HID headlamps). Orange is only allowed for turn signals or trim lights. Green headlights is a maybe, while red headlights, tur signals or trim lights on the front of a vehicle are illegal and are restricted to emergency vehicles only. Check with your local police or department of motor vehicles to see what type of lights are legal in your area.

Q. What are Fog Lights?

Fog lights are yellow tinted lights that cut though fog and haze better than white lights during rainy, damp weather. Yellow light actually produces less reflection and glare than white or bluish light when there is a lot of moisture in the air.

Q. What are daytime headlights?

Daytime headlights are headlights that remain on when ever the vehicle is driven, day or night. To extend bulb life, daytime headlights may be run at 50 percent of normal brightness. The purpose of daytime headlights is safety by making your vehicle more visible to other drivers and pedestrians.

LED headlights 2021 GMC truck

Q. How to LED headlights differ from HID or halogen headlights?

LED headlights, taillights and trim lights are solid state light emitting diodes. They have no filament and no glass bulb. The surface of the LED glows brightly to produce visible light when the LED is on. Various coatings are used to modify the color of the light. LED lights consume much less power than incandescent or halogen headlights, and they will usually last the life of the vehicle.

Q. Do LED headlights run hotter or cooler than halogen headlights?

It depends where you make the temperature measurement. The temperature of the tungsten filament inside a halogen headlight is around 4600 degrees, while the surface of the bulb itself may be 160 degrees F or higher. It's certainly hot enough to burn your fingers! By comparison, the front surface temperature of a LED headlight may only be half as much at 70 to 80 degrees F. But the backside of the LED where the electronics and heat sink are located may be as hot as 100 to 200 degrees F.

Aftermarket LED headlight conversion kit

Q. Can your replace ordinary halogen headlight bulbs with LED headlights?

Yes, there are aftermarket conversion kits that allow you to replace ordinary halogen headlight bulbs with LED bulbs. The main advantage is that the new LED lights should last the life of your vehicle, eliminating the need to replace the bulbs every few years. Custom LED lights can also provide different colored lighting (whiter/brighter) for better night visibility. LED replacement lights are also available for halogen taillights and stoplights, too.

Q. How is the brightness of headlights measured?

Light output can be measured in "lumens" or "candlepower". Both units of measure are measured differently, but both express the intensity or brightness of the light at a specified distance from the source. The higher the lumen or candlepower rating, the brighter the light source. The brightness has nothing to do with the color of the light itself, only its intensity.

Q. How is the color of headlights measured?

Visible light covers a wide spectrum of wavelengths. The "color" or "shade" of the light produced by the headlights is measured in degrees Kelvin. Lights may vary from 3000 K (yellow) to off-white (4300 K) to pure white (5000 K) to bluish-white (6000 K). Most standard halogen headlights are around 4300 K. Natural daylight is around 5800 K.

Offroad aftermarket headlights

Q. Can "off-road" auxiliary lights be used on the highway?

Government rules limit maximum light output for low beams for highway driving to reduce glare for oncoming drivers. For street driven vehicles, DOT rules limit maximum light output for low beams to 700 lumens, and high beams to 1200 to 1300 lumens. Headlamps marked DOT are street legal in all 50 states. Headlamps that are not DOT compliant because of higher than allowed brightness are NOT legal for highway use. Super bright off-road lights may be so bright that they could temporarily blind oncoming drivers, so do NOT use them unless you are the only vehicle on the road.

Q. How do you aim headlights?

Most headlights are some type of adjustment mechanism so the light pattern produced by the headlights can be aimed at the road ahead to provide the best visibility without blinding oncoming drivers. Each headlight typically has two adjustment screws, one for raising and lowering the headlight, and a second screw for adjusting the beam pattern left or right. You want both headlight aimed straight ahead with the beam pattern cutting off level with the hood of your vehicle. Avoid aiming the lights too high or too low. Also, you do not want the beam pattern spreading too far too the left in oncoming traffic.

aiming headlights

Click Here for more info on how to adjust your headlights.

Q. Do some vehicles have better headlights than others?

Yes. Recent testing by government and independent testing facilities has revealed that many vehicles have headlights that deliver less than optimal performance. According to the Insurance Institute for Highway Safety, more than half of SUVs recently tested have "marginal" or "poor" headlights. The optical properties of the reflectors and lens inside the headlight housing determine how the light is projected from the headlight. If the headlight is not well-designed, it may not project most of its light where it does the most good. Better headlight systems will provide broad, even illumination of the road ahead with both low and high beams.

Q. What are "adaptive" headlights?

These are headlight systems that shift the beam pattern right or left slightly so the driver can see further when turning or driving around a curve. An adaptive headlight system monitors vehicle speed, the driver's steering inputs and a "yaw" sensor to determine when the vehicle is turning or driving on a curved road. Small electric motors in the headlight assembly then move a reflector or the bulb itself so the beam pattern will follow the curvature of the road for better visibility. Some systems can also sense when a vehicle is traveling up or down a hill, and redirect the beam pattern up or down to improve visibility.

headlights More Headlight Articles:

Troubleshoot Headlights

HID Headlights

LED Headlamps

All About Headlights

headlights, taillights, bulbs Click Here to See More Carley Automotive Technical Articles

Be sure to visit our other websites:
headlight FAQ

Carley Automotive Software

Auto Repair Yourself



Scan Tool Help