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Car Lights Dimming and Brightening – Troubleshooting

Last Updated on Aug 16, 2023 By Lillian Kazmierczak

If you’ve your car lights dimming and brightening, there can be many reasons behind this issue; from old bulbs or batteries to corroded wiring to failed alternators.

Our vehicle technicians singled out seven possible suspects that can be blamed and showed how to fix all in this guide.

See what has happened in your case!

Car Light Basics You Must Know

Car Light Basics

A lot of things can contribute to the dimming or brightening of your car lights. Only an in-depth understanding of how the car lights act and react can help detect the issue.


Headlights are powered by the battery when the engine is off. The engine takes over after you start the car.

Ground Connection

A wiring harness links the lights with the engine that’s grounded to the chassis. This electrical circuit will cause a complete cut-off of the lights or flicker them unnaturally when disruption happens.


The alternator comes in along with the voltage regulator to circulate electricity from the engine to power all the electrical components of your vehicle.

Note: Headlights light up when working right or they don’t when something is wrong – the rule is simple. This means if yours are flickering or dimming, the issue lies somewhere else.

7 Suspects of Car Lights Dimming and Brightening

1. The bulbs

It is the obvious component you need to look at when your light is dim. The bulbs might have passed their operational age or they’re just poorly assembled.

A standard bulb comes with an average of 5 years of lifespan while replacements vary depending on their manufacturers. Get a new bulb when you see conflicting brightness on each side.

Sometimes, it’s just deposits of time and dirt commonly visible on the halogen bulbs. The torpid vapor inside a halogen lamp eventually generates a sort of build-up all through the inside of the bulb; blocking light from passing through.

2. Old Battery

Check your battery with a battery tester or go to a mechanic nearby since you’ll often get your battery tested for free.

An aged battery will drain faster and is less efficient in holding the charge. The lights might flicker or faint when it’ll start to recharge as you drive.

3. Corroded wiring

Damaged wiring can cause your car lights to flicker, particularly during the drive. The wires linked to the chassis and headlights are a vital point as this is a part of the circuit that lets power to drift.

The electricity cannot run smoothly if the wire is corroded or damaged. As a result, your headlights don’t get the right volt and don’t shine bright.

It’s not always the bulbs; your lights can go entirely out in case the electricity is completely cut off.

Get a replacement for your ground wire. This is not an expensive fix but it’ll take your mechanic a while to get there and replace it.

4. Pale headlight lenses

The newer models today come with a variety of transparent plastic concealments topping off the lights to safeguard the bulbs against damage. This plastic layer can turn pale or get blurred out because of its exposure to the sun.

The lenses may lose their transparency someday since motorway debris such as sands, pebbles, and other dirt can leave abrasions on the outer surface.

Your headlights can seem hazy, obscure, or faded white even if they’re working okay. You may get a lens replacement or get on with a headlight restoration service.

5. Failed Alternator

The alternator works to maintain the correct headlight luster and supply electricity to just about every component, including the power steering, power windows, windshield wipers, dashboard instruments, radio, and cameras.

It takes the punches from your engine and transforms them into applied electrical energy.

Note: Remember that when you turn the key to the ignition, your headlights should be bright as they’ll still be running off the battery. Have your alternator inspected ASAP if the headlights go a little offbeat when switching over to the alternator.

6. Worn-out alternator belt

Dimming and brightening of car lights is perhaps induced by a worn-out or broken alternator belt. The brightness fluctuates with the gripping and gliding of the alternator pulley.

As all belts lose elasticity and become depleted over time, pull over to a mechanic’s workshop when you know it’s time for a replacement.

7. Overloaded Voltage Regulator

This is the last component you need to check. This device works along with the alternator and controls the voltage frequency throughout the electronics in your vehicle.

Lights will hypnotically glimmer and flicker if your regulator loses control over the amount of electricity being put out.

4 Effective Steps To Fixing Car Lights Dimming and Flickering

1. Replace and recheck the bulbs

Replace the damaged bulbs when you’re certain that they are the cause of the issue. You may want to attach the bulbs to a power source to check if they’re actually failing or not and examine how bright they blaze.

2. Investigate the electrical and wiring unit

If you don’t know a thing or two about cars, it’ll be difficult for you to check all the wiring yourself.

We recommend that you let a pro go through your ground wiring and catch on to the source of the fault and fix it for you.

3. Wash the headlight chamber

If your headlight lenses are scratched or blurry, give it a good wash outside and clean from the inside. It might be a good idea to head on to a garage in case you’re less familiar with this kind of operation.

4. Replace the light assembly

Get a replacement sooner than later when you have issues with your light assembly. Take it for granted and you’re in for some serious trouble.


1. Why do my headlights dim and brighten?

Ans: The common causes are a bad alternator, aged battery, damaged ground wire, failed voltage regulator, or a loose alternator belt.

2. Why do my lights dim when I accelerate?

Ans: This may be due to a low supply of power voltage; you’ll find your answer in the ground wiring unit, voltage regulator, and alternator.

3. Why do my headlights dim when I hit the gas?

Ans: Get a load test on your electrical system because it’s a clear sign that you have voltage drop.

4. How long do alternators last?

Ans:  On average, alternators last for about 7 to 10 years or 80,000-150,000 miles but you may need a replacement sooner if you drive real hard around.

About the author

Lillian Kazmierczak

As far as I can remember, I would say I have been a car nut for my whole life. My father was a car dealer who used to change and repair his cars himself. This gave me the opportunity to get around all sorts of cars and get my hands dirty repairing vehicles from an early age.

A great fan of Japanese quality and German preciosity, my deep passion lies in older models that I believe have a flair that takes me back to my childhood! I also love their extraordinary durability and reliability when compared to today’s modern models.

When not out taking a ride, I enjoy socializing with fellow motorheads online and consuming any car facts and figures I can get my mind on!

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