Drivers rely on their headlights to safely get us around in the dark. A lot of us take our headlights for granted until something goes wrong, and our lights are suddenly dim. This issue can be really annoying and dangerous, so it’s important to quickly fix it.
The biggest part of the repair is the troubleshooting step. You’ll need to figure out what’s causing the dim lights before wasting time and money on repairs that might not even help. It could be the battery, alternator, wires, fuses, components, connections, headlight lens, or a setting that got adjusted without you realizing it. Once you find the culprit, you’ll need to go through the corresponding repair process.
That’s why I put together this ultimate guide. I’m going to walk you through the full troubleshooting process and help you get to the bottom of this. In this guide, I’ll answer some popular questions and I’ll show you how to troubleshoot and repair dim headlights and dim cab lights in two separate sections.
What Causes Dim Headlights?
You’ll see in a little bit, but there are a series of components that could cause dim headlights or cab lights. Both of these parts are electrical units that are fed by the battery.
It could be the components themselves, physical issues, the battery, the alternator, wiring issues, or damage to fuses (to name a few).
The true cause of your dim headlights won’t be found until you start doing troubleshooting in later sections.
The Dangers of Dim Headlights
Dim headlights are one of the most dangerous problems that a car can have. It’s all about visibility.
For one, you won’t be able to see upcoming obstacles on the road if your lights are dim. By the time the object is illuminated, you might not have enough reaction time to get out of the way.
The other issue is that other drivers can’t see you when it’s dark, rainy, or snowy. They might think they’re good to cross an intersection because they can’t see you approaching. On a highway, this could be really bad.
I hope you fix this problem as soon as possible.
How Bad Is This Problem?
This problem isn’t necessarily that bad. It could be something as harmless as a dying lightbulb or dirty lenses. The worst-case scenario is wiring issues, a dying battery, or a dying alternator. All of which aren’t that expensive to fix.
Still, it’s better to troubleshoot and fix this problem now before your car loses power on the highway.
Can You DIY the Repair?
As you’ll see, a lot of the solutions are pretty easy. There are a few complicated electrical repairs, and I’d suggest going to a mechanic for those fixes. However, all the other issues can be done on your own.
How to Fix Dim Headlights on a Car
I’m going to start with a section dedicated to just fixing dim headlights. If you have dim cab lights, please go to the next big section. While these issues might seem very similar, the means of troubleshooting and repairing them are surprisingly different.
Your Parking Lights Are On
I don’t want to be Mr. Obvious here, but you might not have your headlights fully on. I can’t tell you how many times I’ve passed someone on the highway who had their parking lights on, not their headlights.
For reference, parking lights are the much dimmer option out of your lighting choices. They are supposed to be used for cars that are just parked somewhere, so they can be seen without being too obnoxiously bright or wasting too much energy.
Your parking brakes will not illuminate the road the way that you’re used to.
Look around the steering wheel for your headlight setting. Make sure it’s set to “AUTO” or “ON”. If this fixes your problem, then you don’t have to worry anymore.
Replace Aging Headlight Lenses
The headlight lens is the glass piece that goes in front of your headlights. It protects sensitive components inside the headlights from weather, road debris, and damage.
The only problem is that these lenses don’t age gracefully. If you have an older car, you might notice that your lenses are yellowed. While this might just seem like an aesthetic issue, it will certainly dim the output of your headlights.
Keep in mind, all the light generated from your headlights has to pass through these lenses. The foggier and more discolored they become, the less powerful your headlights are.
Replacing the lenses are pretty easy. Start out by finding OEM replacements. I would suggest against aftermarket lenses because there’s no telling how good the parts are. If they use cheap materials, the new lenses will discolor and fail quickly.
To remove the lenses, start by taking out the headlight assembly. This can be done by prying and disconnecting each of the lights. On the backside of the assembly, you’ll see some fasteners which clip the lenses in. Undo these fasteners and pop off the lenses.
Replace them with new lenses, reattach the fasteners, and put the assembly back into your car. Don’t forget to plug in the lights.
Clean Dirty Headlight Lenses
If your lenses are just dirty, there’s a way to save them without buying replacements. I’ve done this before on my older cars when I was younger.
You don’t need to disassembly anything for this to work (hopefully). I recommend a specialized product like the one from 3M that I use. It comes as a kit with the instructions printed on the back.
Start by using soap and water on the lenses and surrounding bodywork. Use painter’s tape to mask off all parts of your car’s body around the headlight lens. Use water on the gray side of the polishing disc (which comes in the kit).
Use circular motions with a little pressure to sand away the discolored part of the lenses. Make sure you keep applying water to the disc to keep it wet.
Dry the area, check to see if the discoloration is gone and redo the process as needed.
When the discoloration is gone, use the packaged cleaner. Apply a quarter-sized blob on each lens and rub it in aggressively with a nice microfiber towel. Keep doing this until all the haze on your lenses is gone.
If the haze or discoloration won’t go away, you might need to revert back to replacing the lenses with new ones.
Change the Bulb
Just like the lights around your home, the lights in your car will eventually die. However, they don’t have to necessarily die overnight — they can slowly fade and become dimmer until they finally go out.
The obvious way to tell if your bulb is dying is by turning on your lights and standing in front of your car. If one headlight is a lot dimmer than the other, then the dim one probably needs to be replaced.
Start by turning off your car. Pop the hood of your car and look near the front to locate your headlight bulb. There is a wire attaching to the headlight assembly. It probably has tabs on either side that need to be pulled back before completely unplugging the wire.
Next, grab that base that you just unplugged the wire from. The base will pull straight back towards the car’s engine and reveal that it’s actually the base of your headlight bulb. The bulb will be removed by doing this.
Take the bulb assembly over to your desk, and put it down, noting the configuration within the base. Before removing the old one, take your new one out of its packaging. Don’t touch the glass part, especially not with a bare hand. This could create an oily spot on the glass which can heat up and prematurely blow out the bulb.
Position the new bulb so that it’s in the correct position to go into the headlight base. Remove the old bulb and put in the new one.
Put this assembly back in your car where you removed it from, lock it in position, and reattach the plug.
Start your car up and take a look at the headlights. If they still look dim, something else is wrong.
Replace the Ground Wire
There’s a wiring harness that I’ve mentioned a few times. This connects your car’s battery to your headlights.
Like a lot of wires in your car, this harness has a ground wire. Somewhere, the ground wire will touch some exposed metal of your chassis to create a grounded connection.
If you did some DIY electrical alterations to your car recently, you might have disrupted or damaged this ground wire.
Start by trading the wiring in order to find the ground. Check for corrosion, an improper connection, or damage to the actual wire.
For the latter two conditions, you’ll need to completely replace the ground wire. If you’re not electrically confident, then it’s a good idea to have a mechanic do this, instead.
For corrosion, you might be able to use a rust remover and let it dry. If this doesn’t fix it, then I’d suggest using a mechanic.
Why do I suggest a mechanic? Altering your car’s electrical connections can be really dangerous. There are live wires running under your car’s hood, and enough voltage flowing to do some real damage. It’s always best to let a professional do the work, that way you don’t risk burning out components or starting an electrical fire.
Swap Out Your Alternator
Your alternator is the sidekick that your battery needs. Basically, the alternator is used to constantly recharge your battery while you drive around. Since you don’t plug your Camry into the wall at the end of the day, the alternator is the only way to recharge it.
When the alternator fails, your battery becomes less powerful. As a result, all the electrical components around your car start underperforming.
Suddenly dimming headlights is a clear sign that you’re having electrical issues. Couple that with warning lights, stalling, or a sudden loss of power, and you can be pretty sure something’s wrong with the alternator.
I have a guide for diagnosing the alternator bearing here, so give that a quick read. It will walk you through the full troubleshooting and repair processes.
Replace the Alternator Belt
Outside of the alternator is a belt. This connects the driving shaft of the engine to the alternator which then feeds the battery.
Actually, it’s a lot more than just an “alternator belt”. It’s called a serpentine belt and it ties together five or more mechanical parts of your car, under the hood.
If this belt is slipping, stripped, failing, or broken, then your alternator won’t be getting power from the engine. It relies on this power to spin a shaft which generates a recharge for your battery.
Pop the hood and locate the big pulley that twists around multiple shafts. If you notice any damage to it, then replace it. If you’re having troubles diagnosing or locating it, have a trusted mechanic help out.
Swap to LEDs
There is also a newer lighting technology that allows your car to be brighter and gives you better visibility. LEDs are being installed in new cars to achieve better lighting than old halogens.
However, there’s a very important alternation you need to make. Your headlight housing is set up to accept low-strength halogens. If you just throw an LED bulb in there, you’ll nearly blind oncoming drivers since it will be way too bright.
If you go down this path, you need to install the conversion kid to make your headlights compatible with LED bulbs. I say this as someone who has passed countless cars who haven’t done this conversion. It is incredibly bright and distracting and can lead to accidents (as well as tickets if a police officer sees your lights).
You might also have to tweak the directional position of the light assembly. By angling it further downward, towards the road, you have a lesser chance of distracting oncoming drivers.
Check the Headlight Computer Modules
Many vehicles today are equipped with Xenon, HID, or LED headlights which means there is a computer module either directly behind the headlight unit itself or there is a central computer that controls both.
Depending on the year, make, and model of your vehicle, it’s possible that this computer module has gone bad. If there is one on each side, start by swapping them out (if possible) and see if the dimming issue on a specific headlight goes away.
If both headlights are dim and there is a separate computer module for each headlight, then it’s unlikely that both are bad unless it’s a manufacturing defect for them to both fail at the same time.
Replace a Dying Battery
If your alternator is healthy, it could be a dying battery. Remember, the battery is the heart and brain of all electrical processes that go on within your car.
If the battery is on its last leg, then your headlights and all other lights are going to become dimmer before finally dying.
There’s a cool battery testing kit that tells you whether your alternator is bad, or your battery is bad. That’s part of my 8 ways to tell if your battery is dying guide, so give that a read.
Replacing the battery can be done on your own. You’ll need a special battery holder since they’re pretty awkward to move around since they’re so heavy.
Get a Jump-Start
If the battery tester says that your battery life is okay, not bad, then you could benefit from a jump-start. This only works if your alternator is in good health.
If you get a donor car, use my guide to safely jump your cars. If you want to do the job solo, then you can pick up a portable car battery jumper. It charges up like a phone’s battery and can sit in your trunk until you need it.
The portable jumper comes with the cables attached. You simply plug it onto your battery, give it some time, then start up your car and drive away.
After the first jump-start, you should drive your car for at least 15 minutes. It’s okay to drive around the block multiple times, it doesn’t have to be at highway speeds.
This duration of a drive gives your alternator a big enough load and enough time to give your battery a decent charge.
If you went a while without driving your car, this could be the solution.
Check The Terminals of Your Battery
Another thing to check is the terminals of your battery. If they are corroded, show signs of wear, have any level of damage to them or have a liquid on them, then your battery is damaged.
You’ll need to carefully clean off these terminals in order to bring your battery back to life.
Damage to the terminals will slow down the rate at which your battery can give electricity to different components. Your headlights might be getting barely enough juice thanks to these damaged terminals, so they’ll look dimmer.
This is also a good opportunity to check the terminal connections to ensure they’re snug.
If you can’t clean the battery terminals, you might need to replace the whole battery.
Replace a Melted Headlight Connector
Finally, there is a little connector right before your headlights that functions as the final electrical point before the bulb. You might have removed this earlier if you were diagnosing your bulb or headlight assembly (to clean or replace the lenses).
This connector has a series of wires that feed into it. It might also have some built-in sensors and switches, depending on what features your car has.
If there are any black or brown spots on the connector, then it’s been melted. This is more common on older cars because the headlights create a lot of heat which can lead to a small electrical fire in this connector.
Replacing it is easier said than done. You can buy a replacement part, but you might find it difficult disconnecting wires and re-running them.
If you do this job on your own, make sure you disconnect the car’s battery before doing anything. However, I’d suggest having a mechanic take care of this. Supplying them with an OEM replacement might be a good idea, that way you know they aren’t using a cheap aftermarket option.
How to Fix Dim Cab Lights on a Car
In this section, we’ll look specifically at your car’s interior cabin. This could refer to overhead lights, dash lights, or lights near your fold-down vanity mirrors. You’ll notice a lot of differences between this section and the previous one about dim headlights.
Turn Them Up with a Settings Adjustment
I’ll explain the simplest and most obvious solution first. I also want to say that I’ve personally had this happen to me before, so don’t be embarrassed.
Somewhere in your car, there’s a way to manually change the brightness of your dashboard lights. For my car, there’s a dial next to the steering wheel. In my previous vehicle, it was a setting that I had to cycle through in my car’s dashboard controls.
If you don’t know where it is, then you should peek at your owner’s manual or do a quick Google search.
This setting will make your dash lights dimmer. If someone just detailed the interior of your car and accidentally hit the dial or switch, then your lights might be suddenly much dimmer.
Use this setting to turn them back up and make them brighter for you. If this fixes your problem, you can take a deep breath. There are no electrical issues or expensive fixes that you need to worry about.
Check That They’re Turned On
Another easy fix is checking whether or not the light switch is on. Again, this is something that could have been accidentally hit when someone was in your car.
Near your dome lights, there is usually a switch. Toggling it will either turn the lights on permanently, off permanently or toggle them to turn on when a door is opened.
Play around with this switch and see if you can revert to the option that you previously used.
Older cars might have this switch hidden somewhere near your steering wheel. This is another opportunity to look at your owner’s manual and figure out where this toggle switch is for your dome lights.
Replace Dim Lights
Just like your headlights, the dome lights and vanity lights can become dim. This happens when they’re dying.
Since interior lights are typically LEDs, it’s more common for them to suddenly die or flicker rather than lighting simply being dim. However, I wouldn’t rule this out since it’s such an easy fix.
If only one or two of your interior lights are dead, then it’s probably a matter of blown lights. Remove the plastic covering from the light. That will expose the housing in which the lights are installed. Remove the bulbs while noting their orientation.
Replace them with OEM replacement lights to ensure the brightness and shade match with the rest of your interior.
Check For a Loose/Faulty Wire
There will also be ground wires associated with your cab lights. Tracing them is a little tougher since a lot of it is done behind different panels.
The only way to troubleshoot a faulty wire is to pull away some of the trim around your ceiling and track down the wires.
If you see any damaged wires, you’ll need to pull them and re-run them. This is another time I would suggest getting a mechanic involved instead of trying to DIY.
Inspect the Light Control Module (LCM)
Just like there are often times computer modules for headlights, there is usually a dedicated light control module for the interior lighting. If the LCM is faulty, it’s more likely that the interior lights wouldn’t work at all.
Dimming issues in the vehicle’s interior cabin would usually be caused by other factors. It’s rare but possible that a faulty LCM is causing intermittent or dim interior cabin lighting.
Replace the Blown Fuse
Having a blown lighting fuse is rare unless there is some other bigger issue. For reference, a fuse is used as a sacrificial piece. Instead of your light blowing out thanks to a surge of power, a fuse blows out.
You’ll have a fuse box hidden somewhere in your vehicle. You’ll need to check your owner’s manual to find out exactly where, but there’s a good chance it’s behind your glove box.
Take a look at the fuse box and search for blown fuses. If you see any, replace them with the correct alternative. If there’s a fuse map near the box, then check for which fuse is responsible for your cab lights to narrow down your search.
Replace a Dying Battery
A dying battery might also be the reason why your cab lights are dim. It typically also leaves you with dim headlights, so you probably fixed it in the previous section.
Either way, diagnosing the battery is just a matter of plugging in a tool and seeing how healthy your battery is. If it needs to be replaced, then you can unplug the battery, pull it out, and install the replacement battery.
It should give you an immediate fix to your problem.
Replace a Faulty Door Jamb Switch
When your cab lights are healthy, they’ll turn on when you open your doors. That’s because there’s a switch in your door jamb which senses that the door is opened, then sends a “go ahead” signal to all the lights.
In fact, most of your interior lights will probably get wired into your door jamb. Open your driver’s side door and locate a little rubber cover. Pull the rubber and you’ll pull out a boot-shaped piece with wires running to the back of it. This is boot-shaped piece is the switch.
Make sure your car is turned off and the battery is unplugged. Unplug the wires running to this boot and replace it with your new door jamb switch. That should fix things.
Dim headlights and cab lights can be dangerous. The good news is that a lot of the causes can be fixed pretty easily. Leave a comment below if this ultimate guide answered your questions and helped you get your lights back to their full strength.