Bright vs. Dim: Why Is One Headlight Dim & the Other Bright?

Car headlights are designed to last anywhere between 500 and 1,000 hours. However, there are a few issues that can force you to replace them earlier on. 

One such problem is when one headlight becomes dimmer than the other. 

This is typically caused by a corroded ground connection, cloudy lenses, worn-out bulbs, misalignment, or poor installation of the headlight

The following post provides a more detailed answer to the question, “Why is one headlight dim and the other bright?

What Would Cause One Headlight to Be Brighter Than the Other?

Have you noticed the headlight on the driver or passenger side casting a dimmer light than the other?

If you have, this is likely being caused by one of the following:

1. Worn Bulb

If you don’t switch on your headlights regularly, they can last for years before having to be replaced. 

However, if you frequently drive with your headlights on, then one of the bulbs can wear out sooner than the other. 

This wearing out happens because the filament inside the bulb vaporizes, leaving behind a gray/brownish metallic residue in the headlight capsule.

This residue reduces the light output considerably, which is why the headlight bulb becomes dim. 

2. Foggy Lenses

If there’s one thing that many drivers don’t know, it’s the fact that a dim headlight can also be caused by a cloudy lens. 

Lenses – which are the plastic components used to cover the bulbs – are usually made of acrylic. 

Unfortunately, this material oxidizes over time, creating an opaque layer that resembles fog. This opaque shade doesn’t allow as much light to pass through.

So you’ll notice one headlight being brighter than the other. 

3. Incorrect Headlight Assembly

Have you noticed one headlight becoming dimmer after a recent replacement or DIY fix associated with the headlights?

If you have, there’s a high chance that it’s being caused by improper installation. 

Perhaps you didn’t position the bulb correctly inside the headlight housing. As a result, it’s not able to project light correctly, which is why it looks brighter than the other. 

4. Corroded Ground Connection

Other drivers attribute dimmer headlights to poor power connectivity. However, this is rarely ever the problem. Rather, the most likely culprit is the corroded ground connection. 

Just in case this sounds unfamiliar, the ground circuit is a short wire connected to the chassis just close to the headlight. 

It’s responsible for creating a path for electricity to flow from the negative terminal on the battery to the headlights. 

In most cases, the vehicle delivers more than enough power to the headlight system. 

But if the ground connection is corroded, the bulb won’t emit the same amount of light as the properly functioning one. 

Given the complexity of this issue, it’s best to hire a professional mechanic to handle it. 

5. Wiring Issues

Even if ground circuit corrosion isn’t the culprit, other wiring problems can interfere with the light output.. 

Here’s the deal: car headlights rely on multiple components to illuminate. 

Two things that play a particularly important role in this mechanism are a wiring harness and fuse. 

The wiring harness supplies power from the car’s battery directly to each headlight. Meanwhile, the fuse protects the bulbs by breaking the circuit when it detects too much power flowing

If any of these parts are damaged or broken, then it will cause one or both of the headlights to dim or cease working altogether. 

Is It Illegal to Drive With One Dim Headlight?

Yes, it is. Just because you can drive with one dim headlight bulb doesn’t mean it’s okay to do so.

In fact, drivers caught driving with just one functioning headlight are likely to be pulled over and fined; and rightly so. 

This is because it endangers your safety and that of other road users. 

When you have one light dimmer than the other, drivers approaching you from a distance will perceive your car as a motorcycle.

This means they won’t give you the right clearance, increasing the risk of an accident. Plus, you’ll have lower visibility and might end up hitting an obstacle on the road. 

What Should I Do if One Headlight Is Dimmer Than the Other?

If you notice any of the lights in your vehicle are dimmer than the other, you should get the issue resolved immediately.

Here are a couple of remedies you can try:

1. Adjust the Alignment of the Lights

If you were involved in an accident recently or performed a DIY fix on a different part of the headlight housing, it’s likely that you didn’t install the headlight bulbs correctly. 

This is an easy problem to fix. 

Simply, remove each part of the headlight assembly and reinstall it the right way. 

Pay more attention to the bulbs positioning, ensuring that each one is aligned properly. While you’re at it, double-check the headlight settings in your vehicle. 

Switch on the high beams and low beams separately and observe their brightness. If they’re working properly, it means the dim headlights were being caused by misalignment. 

2. Replace Worn Bulbs

Has one of the bulbs lasted significantly longer than the other? If it has, it’s likely that it’s burned out or worn; hence, causing the dim lighting. 

Like the alignment issue, this is pretty easy to fix. Simply, buy a new bulb to replace the old one and you’re good to go! That said, be careful when choosing replacement bulbs. 

If you buy a different type, you could end up with the same issue. This brings me to my next point.

3. Verify the Type of Bulbs

Another problem that causes one headlight to be brighter than the other is if it’s a different type. 

There are two main types of headlight bulbs: LEDs and incandescent. The LED varieties are much brighter due to their intense, white light with bluish accents. 

On the contrary, the traditional incandescent lights produce a softer, warmer glow with a slightly yellow hue. 

Recently, LED headlamps have been gaining more popularity, and for good reasons. For one, they last much longer than incandescent varieties. 

Secondly, they’re more energy-efficient. However, these lights also produce a sharp glare, which can be blinding for oncoming drivers. 

If you’re planning to replace your car headlights, be sure to replace them with the same type. 

This way, you can maintain uniform brightness. 

If you can’t find a replacement that’s the same type as your current one, then get two brand-new bulbs – as long as they’re the same type. 

How Much Will It Cost to Repair?

The cost of repairing car headlights depends on the nature of the problem at hand. 

For instance, if it’s a misalignment issue, then you can solve this by yourself and save a few bucks. 

Just take the entire headlight housing apart and reinstall each component correctly. 

However, if you’re dealing with wiring problems or other complex issues that warrant a replacement, you’ll spend significantly more. 

Generally, the cost of replacing headlight bulbs and the labor involved in installing them can cost anything between $30 and $300


Why is one headlight dim and the other bright? If this question has crossed your mind at some point, then this post will help you. 

The most common causes are faulty ground wire, improper headlight installation, misalignment of the bulbs, worn bulbs, and wiring issues. 

Depending on the underlying issue, the dim headlight can be resolved by replacing it with a new one or realigning it so that it projects light properly.


Dean Alvarez, TireForge Head Author

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