Headlight technology isn’t very complex, but there are many different ways that headlights can fail. So if you find that your headlights have suddenly stopped working, it’s important to take note of the type of fault you’re dealing with and move on.
The troubleshooting process you follow will depend on the type of failure you are dealing with. With that in mind, it can be extremely helpful to start by considering whether both or just one of your headlights have failed, and whether the high beam or low beam mode is working.
Common Situations and Fixes for Headlights Not Working
When headlights stop working, it’s usually an electrical problem or a physical problem with the bulbs themselves. In order to get to the bottom of the situation as quickly as possible, it is important to note exactly what type of failure you experienced.
Depending on which light bulbs have stopped working, and under what circumstances, you can use the following information to narrow down the solution:
One headlight does not work.
- Cause : This is usually caused by a burned out light bulb.
- Correction : Replace the light bulb. If it still doesn’t work, suspect a wiring or fuse problem
High Intensity Intensity (HID) headlights can also fail due to other related components.
None of the headlights work.
- Cause : Burnt out light bulbs or a power or ground problem.
- Correction : Check for power and ground, and correct if necessary. Otherwise, replace the bulbs.
Bulbs don’t usually burn out together, but it’s still important to rule this out by checking the wattage. Most complete headlight failures are caused by a failed component such as a fuse, relay, or module. Wiring problems can also cause both headlights to stop working.
The high beam headlights do not work or the low beam does not work.
- Cause : Bulb burnt out or problem with high beam switch or relay.
- Correction : Replace a light bulb, switch or relay.
If only one bulb is not working in either high or low beam mode, it could be the bulb. Most headlight failures that are limited to either high or low beams are related to the high beam relay or switch.
Headlights work but seem dim.
Cause : Foggy lenses, worn out bulbs, or a problem with the charging system.
Correction : Clean lenses, replace light bulbs, or repair charging system.
If your headlights are always dim, foggy lenses or worn bulbs could be the problem. If under certain circumstances your headlights dim, there may be a problem with the charging system.
Other headlight problems are also caused by a combination of faulty bulbs, wiring or relay problems, and bad switches.
How do headlights work?
Most headlight systems are quite simple and include a few basic components such as bulbs, a relay, a fuse, and a switch. There are variations on this basic theme, for example some vehicles have daytime running lights, adaptive headlights or other small folds such as fog lights but the idea remains the same.
When you turn on the headlights, this switch will activate the relay. This relay, in turn, actually provides the electrical connection between the headlight bulbs and the battery . Fuses are also used to provide a sacrificial point of failure to protect the rest of the wiring.
In the same way that your headlight switch activates a relay to supply power to your headlights, controlling your high beams will usually activate a relay to turn on your high beams. In the case of dual filament pods, the headlight literally transfers power to the headlight.
If any of these components stop working properly, your headlights will not work. And by looking at how they failed, you can usually go back to figure out where is the best place to start troubleshooting.
Fix it yourself or take it to a mechanic?
Fixing a blown headlight is usually pretty easy, but there are times when you may want to take it to a mechanic. If you don’t have basic tools and diagnostic equipment such as screwdrivers and a voltmeter, then you might want to consider taking your car to a pro during daylight hours.
If you drive to the store, they will probably start a visual inspection of the headlight system, check the fuses and look at the switch and relay.
Replacing a burned out headlight usually only takes a few minutes, but the diagnostic procedure can take half an hour to an hour, or even more if you’re dealing with a more complex problem.
The diagnostic procedure that a professional technician will actually follow is similar to that described below. So if you want to know more about what to expect when you get in your car to get your headlights fixed, you can continue reading.
Fixing one bad headlight
When one headlight stops working and the other works fine, the problem is usually a burned out bulb. Even if both incandescent bulbs were exposed to the same conditions, they usually do not fail at the same time. So it’s actually quite common for one light bulb to burn out before the other.
Before writing off an incandescent bulb as bad, it’s important to check the electrical connector for signs of damage or corrosion. If the connector is disconnected, installing it may solve the problem. However, you’ll still want to dig a little deeper to understand why it failed in the first place.
Another factor to consider before replacing a burned out headlight capsule is whether or not there are external causes for the failure. Ordinary halogen capsules can last from 500 to 1000 hours . So if yours didn’t last that long, there might be another problem at work.
It is easy to look for any water or condensation inside the headlight assembly. If the seal is worn or damaged, or the housing itself is cracked, water can easily get in. When this happens, the life of your headlight capsule will be seriously compromised and the only fix is to replace the headlight assembly.
Additional problems with hidden headlights
Traditional halogen headlight failures tend to be fairly simple, but things can get tricky when you’re dealing with xenon or HID headlights . While a HID bulb can burn out, there are a few other potential points of failure that you should look out for. The bulb may have burned out, or the problem may be due to a bad igniter or wiring fault.
The easiest way to check if the HID lamp capsule is working is to carefully remove both lamps and replace the one that is not working with the one that works. If a known-good light bulb doesn’t turn on when you plug it into a different outlet, you’re dealing with a more complex problem.
It is important to note that if you are changing bulbs to rule out an igniter or wiring harness problem, you should avoid touching the glass shell of the capsule. Any oils or other contaminants from your hands or anywhere else will greatly shorten the life of the lamps.
If you’re not entirely confident in your ability to change light bulbs without contaminating your glass bulb, don’t do it. You can destroy or drastically shorten the life of your good light bulb.
What to do when both headlights stop working
When both headlights stop working at the same time, the bulbs are usually not at fault. The main exception is when one headlight first burns out, goes unnoticed for a while, and then another lamp fails.
If you suspect the bulbs may be bad and you have a voltmeter, you can start the troubleshooting procedure by checking for power to the headlights. The best way to do this is to turn on the headlight switch, connect the negative lead on the gauge to a known good ground, and touch the positive lead to each headlight connector terminal.
One of the terminals should show battery voltage, and the other two should show nothing if the problem is burnt bulbs. You can then try to activate your far beams, which should cause the other terminal to show battery voltage. If so, then replacing the bulbs should solve your problem.
Checking fuses, relays, switches and other headlight circuit components
The first and easiest component to check is the headlight fuse. Depending on how your headlight circuit is set up, there may be one or more headlight fuses. If you find a blown fuse, replacing it may fix the problem.
When replacing a blown headlight fuse, it is important to use a new one with the same amperage rating. In the event that a new fuse is blown, this indicates a problem elsewhere in the circuit, and replacing a higher amperage fuse can cause catastrophic damage.
If you find that the fuse has not blown, then the next step is to check the power with a voltmeter. You should find the battery voltage on both sides of the fuse. If you don’t, then you need to look at the wiring between the fuse box and the battery.
The next step is to find and test the headlight relay. If you pull the relay and shake it and you hear something rattling inside, then it is probably bad. A change in color on the base or terminals may also indicate a problem.
If you’re lucky, you’ll find that the same relay used in your headlight circuit is used in one or more other circuits. In this case, you can easily replace the headlight relay with an identical component. If at this point the headlights start working, then the problem is in the relay.
In addition, the diagnostic procedure is a bit more complicated. To determine if the relay or switch is faulty, you will need to check if the relay is receiving power when the headlight switch is on. If it doesn’t, then there is a problem with the headlight switch or the wiring between the switch and the relay.
If your vehicle has a headlight, daytime running light, or similar component, diagnostic procedures can be even more difficult. In these cases, the best course of action is to exclude all other component in the first place.
How to Fix Low or High Beam Headlights Not Working
Many of the same problems that can cause your headlights to stop working altogether can also cause only low or high beams to fail. If you find that only one bulb turns off when you turn on the high beam and the other works just fine, then the high beam filament has probably burned out in the first bulb. The same is true if one bulb is running at high beams, but now at low beams.
Most of the time high or low beam failure is due to relay or switch problems and the troubleshooting procedure is the same as above. The difference is that some cars have a separate relay for high beams only, and high beams, low beams, or low beams may or may not turn on in the headlight switch.
If you locate the high beam relay and find that it is not receiving power when the high beam switch or dimmer is activated, then the problem is either with that switch or the wiring. In some cases, this issue can be due to a loose stem switch, although it is more common to find that the switch has failed altogether.