Why is one headlight not working? (Causes & Solutions)

Driving with a malfunctioning headlight is a situation no driver wants to face, as it not only poses the risk of a fine but also endangers safety. For the issue of one headlight not working, there are many possible causes. Therefore, this article aims to thoroughly examine the common reasons for one headlight failure and provide solutions to help you quickly and accurately diagnose and address the issue, ensuring you can effectively deal with the problem and resolve any headlight malfunctions promptly.

Before we delve into the potential causes of a malfunctioning headlight, it is important to have a basic understanding to headlight systems operation. So that we can understand why those reasons may be the culprit of a headlight not working.

How do headlights work?

Most vehicles have a relatively simple design for their headlight working system, of course, some modern vehicles will have adaptive headlights or a design where the headlights share the same bulb as the daytime running lights. But it basically involves a car battery, fuses, a light switch, relays and bulbs. When the headlight control switch is activated on the car signal lever or panel, it triggers a relay that allows the flow of electricity from the battery to the headlights, enabling headlights working. During this process, the fuse will act as a supervisor and protect the normal circuitry, in the event of excessive current or a short circuit, the fuse will blow itself and stop the whole system working.

Therefore, there are the following possible causes of one headlight not working.

The causes and fixes of one headlight not working

Burnt-out headlight bulb

Headlight bulbs are affected by various external factors during use. The filament may break due to the vibration of a car driving on an uneven road or the interaction between the temperature change of the weather and the heat emitted by the bulb when it is working. In severe cases, the glass outer tube of the bulb may even break, which may further cause the bulb to burn out. (At this point, the LED bulb burned out performance is the chip damage and affects the brightness of the light or directly does not light)

• Diagnosis 

First, visually inspect the bulb through the headlight assembly casing to judge if there are obvious burnout symptoms, such as damaged filament shell blackened, etc.

If it is not visible, find the installation location of the problem bulb, take out the light bulb and twist it slightly to check whether the light bulb is removable, and check whether the pin connection terminal is damaged or corroded.

Of course, you can also directly test the bulb with useful tools like multimeter or choose to find another good bulb of the same size and insert it into the bulb socket to verify whether it is a problem with the bulb.

• Solutions

Replace with new headlight bulbs of the correct wattage and size compatible with the vehicle.

• Preventive suggestions

Headlight bulbs will gradually become dim and fragile with the growth of use time. Check the status of the bulbs regularly and replace aging bulbs in time. It is recommended to choose durable LED headlight bulbs, which can reduce the frequency of burnout.

Wrong bulb size or loose connector

If you have just replaced a new light bulb but it still does not work, the bulb size you replaced may be inconsistent with the original vehicle headlight bulb size, causing the vehicle to be incompatible and unable to respond. Additionally, the bulb's socket is not properly secured or plugged into the connector in place, and loose contacts can also cause the bulb to make poor contact with the circuit and not respond. In this case, you may find that pressing it solves the problem.

(Tips: If you're not sure you have the correct replacement bulb size, here's a guide to help: HEADLIGHTS REPLACEMENT GUIDE: WHAT HEADLIGHT BULBS DO I NEED)

• Diagnosis

Verify if the new bulb matches the size of the old bulb or if it is a compatible size that can be interchanged for use in the vehicle. Next, check for the connections if loose.

• Solutions 

Replace with new headlight bulbs of the correct wattage and size compatible with the vehicle and fix connector.

• Preventive suggestions

When replacing a new headlight bulb, be careful not to touch the outer glass or chip of the bulb with your fingers to avoid the new bulb burning out again soon.

(Tips: For general operations and related operating precautions when installing headlight bulbs, please check this blog: HOW TO INSTALL AUXITO Q16 SERIES H11 LED HEADLIGHT BULBS)

Bad or corroded headlight socket

After a bulb has been used for a period of time, the socket's protective coating may degrade or wear away. And more or less moisture will enter and accumulate inside, and the metal will be easily oxidized and corroded. In addition, there are other factors such as electrical problems that can accumulate excessive heat in the socket and accelerate the corrosion process. If not dealt with in time, it will eventually cause the light bulb socket to melt and fail.

• Diagnosis

First use a suitable electrical contact cleaner to clean the socket corrosion, then use a test light to test the metal contacts of the socket to check the grounding condition. If the test light does not light up or is unstable, it means there is a problem with the socket.

• Solutions

Promptly replace severely corroded or damaged headlight bulb sockets.

• Preventive suggestions

Check sockets regularly for signs of corrosion buildup and clean them with a suitable cleaner if found. Use dust cover for bulb protection.

Blown Fuse

One of the most common electrical problems that cause a headlight to not work is a blown fuse. It is also one of the easiest to fix. Each headlight has separate fuses for the high beam and low beam. When a headlight bulb is energized and operates with more current than the fuse's allowable amperage, the fuse will interrupt the circuit connection by blowing, thus preventing a short circuit and damage to other related components. If either fuse blown, the light will go out.

However, it is also possible that the fuses themselves may have failed accidentally, for which it is sufficient to replace them.

• Diagnosis

Remove the fuse box cover and look for the location of the fuse box belonging to the faulty headlight on the legend on the back of it.

Next, use a working test light to check the power to both sides of the fuse. If either side does not illuminate when tested by the test light, the fuse can be removed and the metal strip inside the fuse visually inspected for breakage or burned out.

• Solutions

Replace the fuse with a new one of the same amp rating, making sure the new fuse is compatible with the vehicle. Also, be careful not to damage other surrounding components when removing the old fuse with tools.

• Preventive suggestions

Always follow the specified ratings in your vehicle owner's manual or on the fuse box cover and use the correct fuse rating for your headlights.

Bad Relay

Most automotive headlight systems have relays, one for each high and low beam that control the current operation of the headlights and act as voltage regulators. Although it has a long lifespan, as time goes by, its performance will continue to be consumed due to factors such as frequent switches, heat and resistance, and there will always be a day when it fails or suddenly deteriorates. Once the relay is broken, it will prevent the battery power from connecting to the headlights for power operation.

• Diagnosis

Replace the faulty headlight relay with another, less important relay (same specification, different system). If the headlight work, then the headlight relay needs to be replaced.

You can also listen to the clicking sound the relay makes. Relays make a clicking sound when they operate and receive signals, if there is no sound then it may indicate a faulty relay.

In addition to this, using a multimeter to check the resistance reading of the relay can also diagnose if the relay is malfunctioning.

• Solutions

Replace the relay with a new one, paying attention to the relay specifications specified by the original vehicle manufacturer.

• Preventive suggestions

Choose high-quality relays for replacement. Low-quality relays are more likely to malfunction.

Damaged or faulty wiring

If the bulbs and sockets mentioned above are fine, and the fuses and relays are also working properly, then there may be a wiring problem. The simplest phenomenon is that the connection cable is loose, just reconnect it. However, as the vehicle ages, the wires can become frayed and corroded. This can stop the normal flow of electricity to the headlight bulbs and cause the headlights to stop working.

• Diagnosis

Visually inspect the wiring for any visible fraying and exposed wires, then use a multimeter or test light to test the continuity of the wires.

• Solutions

Replace the damaged wiring, then reinstall the bulb and test the headlight. Because of the complexity of the wiring, it is recommended to find a professional car repairman to replace the wiring.

• Preventive suggestions

Check all wiring regularly for any melting or tearing.

Faulty light switch

Although this situation rarely occurs, there may also be problems with the internal parts of the switch module, causing the switch to prevent current from flowing to the headlights, or the headlight system cannot receive the turn-on command and therefore does not light up as expected. Especially when switching low beam or high beam mode, one of them doesn't work.

• Diagnosis

Listen for a clicking sound from the relay when you turn on the headlights. If there is no response, the light switch may be faulty. In addition, when the light switch is switched, the dashboard light flashing may also indicate that there is a problem with the light switch.

Then use a multimeter to further determine the working status of the switch module. This test is relatively complex, so is advisable to left to a mechanic for a professional diagnosis.

• Solutions

Replace with a new light switch. Some vehicles have light switch modules designed on the dashboard that are relatively easy to replace. For traditional vehicles where the headlight switch is designed on the signal lever, replacing it yourself may take some time. So it can be replaced by a professional mechanic.

• Preventive suggestions

Perform regular maintenance on your vehicle, including electrical system inspections and repairs.

Conclusion

Before attempting any troubleshooting, it is important to ensure that the engine is turned off for safety purposes. If none of the above possible factors solve the problem of one headlight not working, it may be caused by other composite electrical problems in the electrical system, such as insufficient power supply from the AC power system, which requires a professional mechanic to accurately diagnose the vehicle. Of course, if you are unsure about the above diagnostic tests, it is recommended to seek professional help.

FAQ

Q: Can I still drive with one headlight not working?

A: No. No matter what the circumstances, driving without a headlight is a traffic violation, and if you are caught by a police officer, you will get a ticket. In addition, a single working headlight results in reduced visibility, which can lead to accidents. It is advisable to address the problem of non-functioning headlights as soon as you notice it.

Q: Why both headlights not working?

A: It may be caused by the fault factors mentioned earlier. In addition, there may be problems with the headlight assembly, such as cracks in the shell, allowing moisture to penetrate into the internal components of the headlight assembly, or the vents are blocked and water vapor cannot evaporate and accumulates on the bulbs, causing both bulbs to burn out. If none of these solve the problem, it is recommended to seek professional diagnosis.

Q: Why are my headlights dim?

A: When the headlights are working but the brightness dims, it is generally affected by a variety of factors. As headlights age, they inevitably lose brightness. Simply replacing new bulbs or headlight components can solve the problem. In addition, it may be related to the lens of the headlight. The lens will become more or less oxidized or dirty after being used for a long time. Cleaning them may solve the problem.

(Tips: For information about the replacement of the headlight assembly and related maintenance matters, you can read this blog: YOUR CONCERNS - HEADLIGHT ASSEMBLY REPLACEMENT)

Q: Does each headlight have its own fuse?

A: Yes, most modern cars have a separate fuse for each headlight, which increases circuit safety and allows for quick identification of the faulty.