Why Is One Headlight Dim And The Other Bright

This article explains why one of your car’s headlights is bright while the other is dim. 

Many car owners complain about one of their headlights being dimmer or brighter than the other. There are a couple reasons for this, as we’ll discuss below. 

But whatever the reason, this problem needs to be addressed quickly to avoid problems down the line. 

One possible reason for one headlight being dim and the other bright is simply a faulty bulb. This is easily remedied by replacing the bulb, while other problems might require a bit more legwork. 

We’ve been testing and reviewing headlight bulbs for over a decade. One benefit of that experience is that we’ve seen every headlight bulb problem in the book, and can help you solve it. 

Keep reading for more information about why one headlight is dim and the other is bright. 

Common Causes Of Dim Headlights

There are a number of reasons that cause one of your headlight bulbs to be dimmer than the other. 

Faulty Bulb

This is likely the reason one of your headlights is dimmer than the other. It is one of the most common, and also one of the easiest to fix. 

A faulty bulb often exhibits dim light. You may even find that this bulb flickers or takes longer to get up to proper operating brightness than the other. 

Headlight bulbs also tend to develop a brownish gray film on the inner surface over time, This causes them to appear dim or blurry. 

Electrical Wiring Problems

Another common cause for one headlight being dim and the other bright is a problem in your car’s electrical wiring. This could be in the form of a blown fuse, frayed wire insulation, or even a bad conductor somewhere in the system. 

When it comes to electrical wiring problems that cause one headlight to become dimmer than the other, the most common issue is a corroded ground wire. This is the wire leading from your headlight bulb to the vehicle body, and it often becomes corroded, causing dim lighting, flickering, and other such problems. 

Fitment Issues

An ill-fitting headlight bulb will also appear dimmer than normal. This could be due to the bulb not being the right size or type for your particular car. If one headlight bulb became dimmer than the other after you replaced both, it is possible that the bulb wasn’t installed properly. 

Excess Condensation In The Headlight

Water and electricity don’t really gel well. Moisture from the air can build up in the headlight assembly and cause issues with the bulb. 

If there is a lot of condensation build up in the headlight, it could also cause the light to appear dimmer because the condensation causes light to diffuse and reflect abnormally. 

How To Fix Dim Headlights

Faulty Bulb

If the headlight bulb is in fact at fault, all you have to do is replace it. 

When changing out your headlight bulbs for new ones, it is always recommended that you replace both, even if one doesn’t look like it needs to be replaced. 

Replacing both ensures that they will have similar longevity, and you won’t have to deal with one headlight bulb being dim and the other bright. 

Electrical Wiring Problems

The solution for this will be a bit harder than just replacing a faulty bulb. In fact, you will probably have to do a comprehensive examination of the entire electrical wiring system of your car, starting with the battery and alternator, to diagnose and fix the problem.  

And the exact solution will vary from problem to problem. For example, you might need to top up the battery fluid, or clean oxidation off the battery terminals. In extreme cases, you might even have to replace the battery altogether. 

If the problem is a frayed wire that is stripped of its insulation, you will have to get the wire replaced. Similarly, if the problem is a blown fuse, you will have to find and replace it with the correct type. 

And of course, if you’ve identified the problem as a corroded ground wire, then the solution is even simpler. Just trace the wiring harness from the back of the headlight assembly to the point where it connects to the vehicle body. 

Here, you should see some corrosion or buildup. Just clean it off with a wire brush, add a little dielectric grease and the headlight should be restored to its brighter output. 

Fitment Issues

An ill-fitting headlight bulb isn’t particularly hard to fix. The first step is to make sure that the bulb you purchased is the right size and type. You can do this by consulting the owner’s manual or your car manufacturer’s website and seeing if the size of bulb you bought is correct or not. 

If the size and type of bulb is correct but the problem still persists, then the problem might be with how the bulb was installed. Remove the dim bulb and reinstall it, making sure that there are no loose wires, the bulb doesn’t stick out of the socket oddly, and that there are no tabs or locks that are left open. 

Excess Condensation In The Headlight

You can have a professional remove the excess moisture from your headlight assembly. Or if you have just a few supplies and a half hour to spare, you can get condensation out of a headlight yourself and save some money in the process. 

We’ve made a whole guide on how to remove condensation from a headlight that you can check out. All you will need is a vacuum cleaner, some length of hose, a hair dryer, and some duct tape.

How To Prevent Headlights From Becoming Dim Over Time

If you want to prevent your headlights from becoming dim over time, it is important to get ahead of the most common causes for dim headlights. 

Ensure Your Car’s Electrical System Is Functioning Properly

If you want to prevent one headlight bulb from becoming dimmer than the other, make sure your car’s electrical system is in perfect running order. Keep up with regular battery maintenance and fix any wiring issues as early as possible. 

This will also prevent other issues such as with your car’s infotainment system, the ignition, and any other lights in the car.

Use High Quality Headlight Bulbs

When buying aftermarket headlight bulbs, make sure to do plenty of research and look for high quality options that won’t cause problems a couple weeks or even years down the line. 

And if you need help figuring out which headlight bulb is best for your needs, we’ve got plenty of buying guides that do just that. 

Prevent Condensation Buildup

It is important to make sure that your headlights don’t experience too much condensation build up. Condensation usually builds up in the headlight assembly when there is a crack in the headlight or one of the vents that is supposed to evacuate heat and moisture out of the assembly is clogged. 

So fix any cracks in the headlight, and make sure that the vents are functioning normally. 

Conclusion

One of your headlights could be dimmer than the other for any number of reasons. These include electrical problems, fitment issues, and even condensation in the headlight. 

Whatever the case, it is essential that you address these issues as quickly as possible, as they might become even more serious later on. 


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Author

Richard Nickleson is the author and owner behind Headlight Reviews. He first started the site as a hobby to share his insights on car parts and specifically headlight bulbs, but it soon ballooned and now he writes on all topics surrounding headlights bulbs. If you've got a bulb question, contact Richard here.