3 Reasons Why One Headlight Dim And The Other Bright?

If you notice that one of the headlights in the front of your vehicle is beginning to dim, there is likely an issue with the circuit. Bad grounding can cause one headlight to dim while the other is bright. The bulb that is dimming is part of the circuit that is affected. You must replace the bulb.

One of the Headlights on my vehicle is dimming. What does this mean?

So you’re driving down the road and you realize that the light from your vehicle is only shining on one side. When you approach vehicles at red lights, you can see that one of the headlights doesn’t shine as brightly as the other. 

What does this mean? Does this mean the headlight bulb needs to be replaced?

There are three main reasons why one headlight would start to dim, but not the other. The first reason is that there is an issue with the circuit running through the light bulb. The second reason is that there are serious electrical issues developing, and you need to get your vehicle to the mechanic as soon as possible.

Also, it could be that one of the bulbs is defective and is burning out before the other one.

A defective circuit is dimming the bulb

Headlights require a lot of wiring because they have connections to the light bulb and the battery. Sometimes, the wires blinking all of these components together become corroded or dirty.

If you’re able to open the headlight compartment by yourself, you can simply wipe away the dirt and grime around the wiring. When you wipe the wiring clean, make sure the vehicle is off. Then, use a napkin or a soft cloth to wipe the wires. The wires will be very thin. Do not pool on the wires until they are taut. The wires must be loose.

If you pull the wires until they are stiff, and then wipe them with the cloth, there is a real chance you could pull the wire so hard it becomes loose. Then you’ll have to reattach the wire.

Also, check the area around the light bulb socket and behind it. Sometimes corrosion can happen behind the socket. Corrosion is likely to happen because of water damage. If the headlight shield is not secure on the rim, it could have small cracks around the rim that allowed small droplets of moisture inside the headlight.

One of the bulbs is burning out

Companies sell headlight bulbs in pairs and manufacturers install them into the vehicle at the same time. Manufacturers do this so they can burn out around the same time. 

When they burn out at the same time, the vehicle owner can have them changed out and replaced in a single visit and not two separate visits. But nothing is perfect and accidents happen. 

Perhaps one of the light bulbs installed in your vehicle is not as long-lasting as the others. It will dim much sooner than the other light bulb. Well, you cannot drive with a defective light bulb while you wait for the other light bulb to die out. 

Depending on the make and model of your vehicle, you may change it out yourself. But certain manufacturers do not make it easy for owners and regular people to switch out the light bulbs in their headlights. 

Then you’ll have to go to a mechanic. The cost for a mechanic to replace the light bulbs in your headlights is $ $20 to $100 per bulb. The $100 price tag is for luxury vehicles like Cadillacs. 

But if you go to Toyota or a Ford, the press to replace both of the headlights should be about 20 to $30 per light bulb.

Other electrical issues that require meeting attention

So you wipe the wires clean, clean the headlight shield, and replace the bulbs, but nothing is working. Well, that means there are significant wiring troubles around your vehicle’s headlights.

The wiring troubles could be because of a corroded wiring harness, a bad fuse, or a complete replacement of all the wires that attach to the headlight. If the fuse is the issue, then that is perhaps the easiest issue to fix. 

Replacing the fuse only requires that you purchase a new fuse, which is usually $2 to $10 dollars for an entire box of fuses, and replace the headlight fuse. You can find the headlight fuse in the fuse box. 

Use your driver’s manual to locate the fuse box. The driver’s menu should also tell you which number fuse corresponds to which part of the vehicle. Once you find the headlight fuse, simply pull it out and replace it with a new fuse. If that’s not the issue, then you will not have to take your vehicle to a mechanic to check the wiring and the wiring harness.

Wipe the headlight Shields

Like any other part of a vehicle, the headlight shield can become grimy and cloudy. If you can access the inside of the headlight shield, wipe the inside and the outside of the shield. 

In a few uncommon cases, the light bulb could be worth keeping perfectly fine, but the glass around the light-producing part has become cloudy. This can happen for several reasons, but it means that the wiring is perfectly fine, but the glass is dirty.

If you need to replace a light bulb, replace the other one. Always replace the light bulbs in pairs, even if the other ones are working just fine. By doing this, the light bulb will burn out at the same time.


Issues with the wiring harness or with corroded wires mean a large electrical repair job is the only thing that will fix the dimming headlight. Take your vehicle to a mechanic so they can replace the headlight bulbs for you if you do not want to do it. If you can access the headlight bulb yourself easily, then you can change out the bulbs for as little as $30.