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Why is One Headlight Dim and the Other Bright

March 1, 2024 | Victor Lukasso

If you’ve ever been driving at night and noticed that one of your headlights appears dim while the other is bright, you may wonder what’s happening.

It’s important to note that headlights may become dimmer over the years due to usage conditions, dirt, or a defective headlight. However, both headlights might not have the same life span, thus resulting in one headlight becoming brighter than the other.

This common issue can have several causes, ranging from a simple weak bulb to a more complex electrical issue. You can refer to our article on how to make your headlights brighter.

In this article, we’ll explore why one headlight may be dimmer than the other and how to fix it.


What does it mean when one headlight is dim?

If one headlight suddenly goes dim, there’s a probability that the bulb is weak, there’s an issue in the ground wiring, or the voltage (battery or alternator) supply to the headlight is low.

Asides from safety concerns, a dim headlight can also result in a failed inspection. Many states require annual vehicle inspections, and a failed inspection can result in costly repairs and fines.

While this issue might resolve itself (depending on the cause), in some cases, you must get to the root cause of the problem and fix it once and for all.

Why is one headlight not as bright as the other?

One headlight is dimmer, while the other is bright because of a weak bulb or a corroded wiring connector. However, other factors contribute to this.

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These include;

Weak Bulbs

Weak, burned-out, or failing bulbs are the most common reason for a dim headlight. Bulbs have a lifespan and will eventually fail over the years, resulting in a dim headlight.

Also, bulbs can damage faster if the voltage is too high and appear dim if they become misaligned or loose.

Corroded Wiring

If you’re confident the bulbs are in perfect condition, check if the problem is from the wiring connection (electrical). The headlight circuit is relatively simple but can be affected by various issues.

The issues can include a corroded wiring connector, a blown fuse, a faulty relay, a weak battery, a damaged wire, or a problematic wiring connection.

Voltage Issues

Low voltage can cause one headlight to appear dim than the other bulb. If the voltage supplied to the headlight is low, it will appear dimmer than the other headlight.

A faulty alternator, a weak or dead battery, or a corroded battery terminal contribute to a dim headlight.

Dirty or Foggy headlight lenses

Over time, the lenses on the headlights can become dirty or foggy. This can cause the headlight to appear dim as the dirt or fog diffuses the light. The problem is more common with older vehicles, as newer vehicles typically have lenses made from more durable materials.

Incorrect headlight aim

If the headlights are not aimed correctly, one headlight may appear dim. The aim of the headlights is essential for both safety and visibility. If the aim is too high, the light will shine into the eyes of other drivers, which can be dangerous. If the aim is too low, the light will not illuminate the road properly.

Different Headlight bulbs

Installing different Headlight bulbs might cause one headlight to be dimmer than the other because one can be brighter than the other. For instance, Led bulbs are brighter than halogen headlight bulbs.

This will make the other bulb appear dim than the other when driving.

How do you fix one headlight Dim and the other Bright?

Now that we’ve seen why one headlight is brighter than the other let’s see possible DIY fixes you can implement to get the two headlights to shine bright.

Also, Attempting to fix the issue without the proper knowledge and equipment can be dangerous and cause further damage to the headlight assembly. If you are unsure of how to proceed, it’s always best to seek the advice of a professional mechanic.

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Aim the Headlight Properly

If the issue is from a misaligned headlight, the solution is to adjust the headlight’s aim.

Most vehicles have a headlight aim adjustment screw that can adjust the aim.

It’s essential to follow the manufacturer’s instructions when adjusting the aim to ensure that the headlights are aimed properly. You can check our article on where is the adjustment screw on a headlight for more clarification.

Replace the Bulbs

If the bulb is either weak, a different type, or burnt out, the possible fix is to replace the bulb. It’s a relatively easy fix that can be done at home with the right tools.

It’s important to note that replacing both headlights simultaneously is a good idea to ensure they have the same brightness and color temperature. However, if you know about the brighter bulb, you can install it on the dim side.

Clean Headlight Lenses

If the lenses of your headlights are covered in mud, dirt, or grim, it will affect your bulbs’ brightness. The solution to this problem is to clean the lenses.

There are several methods for cleaning headlights, including using a specialized cleaning solution, toothpaste, or baking soda.

It’s important to note that some cleaning methods can cause damage to the lens, so it’s essential to research the appropriate cleaning method for your vehicle before proceeding.

We recommend you purchase the headlight restoration kit from Amazon and apply it to your lens.

Check your Battery or Alternator

If the problem is from a low voltage issue, the solution is to check your vehicle’s charging system.

A professional mechanic can use a multimeter to test the alternator’s and the battery’s voltage output. If the issue is related to the battery terminal, it can be cleaned and tightened to ensure a proper connection.

Also, you can recharge the battery if it’s dead or get a replacement if there’s no hope of getting it to work again.

Check your Wiring

Visually inspect if the headlight wiring isn’t corroded or short-circuited. In this case, I’ll recommend you seek the advice of a professional mechanic to help you diagnose the problem.

Attempting to repair an electrical issue without the proper knowledge and equipment can be dangerous and cause further damage to the vehicle.

Check the Headlight Assembly

If you’ve implemented all of the above fixes and the defective headlight isn’t shining bright like the other one, then you should allow a professional mechanic to inspect your headlight assembly.

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A professional mechanic is in the best position to tell you if the cause of the defect can be managed or if you’ve to purchase and install a new headlight assembly.


Why is one headlight not as bright as the other?

If one headlight is not as bright as the other, it depicts that the bulb is weak or burnt-out; there’s an issue with the wiring connection or voltage.

Can a fuse cause one headlight not to work?

A fuse protects electrical components in your vehicle, including your bulbs; if the fuse gets blown, the headlights and most electrical components of your car will stop working.

Can you drive with a dim headlight?

Yes, but it can be dangerous as there’s reduced visibility, thus increasing the chance of accidents endangering your life and other road users. It’s advised to keep your headlights in working condition if you mostly drive at night.

Can battery cause dim headlights?

A weak or dead battery can cause a dim headlight. The battery helps power a vehicle’s electrical components, including the radio, air conditioner, headlights, etc.

What are the most probable causes of one low-beam headlight not working?

Some of the most probable causes of one low beam headlight include a corroded wiring connector, a blown fuse, a faulty relay, a weak battery, a damaged wire, or a problematic wiring connection.

Why is only one high beam not working?

A burnt bulb is the reason why one high beam is not working. I’ll advise that you replace the bulb immediately.

Will a weak alternator cause a dim headlight?

A weak alternator will cause the headlight to start dimming, flicker, or not work. You must check your vehicle’s charging system and keep them intact.

Which headlight is best for night driving?

From experience, the LED and HID headlight bulbs are best for night driving because they shine brighter and go farther than other headlight bulbs.

How often should you replace headlight bulbs?

The recommended period to replace headlight bulbs is when they get weak or cannot produce bright light. This can vary, but 2,000 hours and 3,000 hours are advised.

Do headlights drain the battery while driving?

Yes, headlights drain the battery while driving, but it’s minor compared to that used by an Air conditioner or radio.

Why are my headlights still dim after replacing the bulb?

If your headlight remains dim after replacing bulbs, then the problem is from the wiring connection. You should check if there’s no corrosion or other issue in the wiring harness of the headlights.

How to Prevent Headlights from Dimming

Here are some actionable tips to ensure your headlights last longer.

  • Regularly clean and wash your headlights to avoid clogging with dirt, mud, or grime.
  • Constantly realign the headlight aim whenever you hit an object while driving.
  • Take your vehicle for regular check-ups to detect issues in the headlight wiring or connectors.
  • Use the correct bulb for your car

What causes foggy headlights?

The leading cause of foggy headlights is constant exposure to sunlight. Sunlight will heat the lens and produce heat inside the headlight, thus diminishing the bulb and turning the lens yellow over time.

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Meet Victor Lukasso, the owner of V. Auto Basics. Through this blog, Victor Provides Insights on the latest tips, maintenance, repair, and techniques in the automotive world.