Headlights are part of a vehicle's electrical system, designed to illuminate the road ahead. Here’s how headlight work: when you switch on your headlights, an electrical relay activates, allowing power from the battery to reach the bulbs. This relay functions as a gateway, ensuring electricity flows only when needed. To prevent electrical damage, built-in fuses cut power if a short circuit occurs.

When any component in this system fails, it can lead to the headlights not working. Below, we’ll explore common causes of headlight not working and how to troubleshoot them.

Common Causes of Headlight Failure

Burnt-Out Bulbs

Bulbs are the primary source of light in any headlight system. Over time, headlight bulbs degrade and eventually burn out. Several factors contribute to bulb failure, including:

Blown Headlight Fuse

Fuses protect your vehicle's electrical system from excessive current flow. If a fuse blows, it interrupts the circuit, preventing power from reaching the headlights.

Faulty Headlight Switch or Relay

The headlight switch and relay control power distribution to the headlights. If either component fails, the headlights may not receive power. Signs of a faulty switch or relay include:

Faulty Wiring or Connections

The wiring and connections in the headlight circuit provide power to your headlights. Loose, damaged or corroded wiring/connections won't reliably deliver power, causing your headlights to malfunction or not come on.

Voltage Issues

Low voltage from a weak battery or faulty alternator can affect headlight performance. Symptoms of voltage issues include: dim headlights and flickering lights.

Corrosion in Headlight Units

Buildup of corrosion on the headlight units themselves can interfere with the circuit, preventing power from reaching the bulbs.

How to Troubleshoot Headlights Not Working

If your headlights stop working, follow these troubleshooting steps to diagnose and fix the issue.

Start with the Headlight Bulbs

To check: Turn on your headlights and open the hood. Inspect the large, round bulbs in your headlight housings. If the filaments inside any bulbs are broken or detached, or bulbs look very dark, the bulbs likely need replacement.

To fix this problem, purchase and reinstall the new replacement headlight bulbs that match your vehicle make and model.

Test the Fuses

If the bulbs are intact, check the fuses next. Locate the fuse box, typically under the hood or in the driver's side footwell. Refer to the owner’s manual to find the headlight fuse. Pull out the fuses labeled for your headlights or "HL" to examine the metal strip inside. If broken or melted, the fuse is blown.

To fix: Purchase replacement blade fuses that match the amperage rating on the fuse label. Remove the old fuses and install the new ones. Start your vehicle and test the headlights to ensure the new fuses fixed the issue.

Inspect the Headlight Relay

The headlight relay is responsible for supplying power. Try swapping it with another identical relay (like the horn relay). If the headlights start working, you’ve found the problem—replace the faulty relay.

See If Power Is Reaching the Socket

Turn on your headlights and use a multimeter to check if electricity is reaching the socket. If there’s no voltage, the issue could be with the wiring or the headlight switch.

Check for Wiring and Connectors Issue

Visually inspect all wiring and connections related to the headlight circuit for any that are loose, frayed, corroded or otherwise damaged.

To fix: Tighten any loose wiring/connections. Clean corroded components thoroughly. Replace or repair severely damaged wiring. Once complete, re-test your headlights to confirm the issue is resolved.

Test the Headlight Switch

A faulty switch may be to blame. Use a multimeter to check for continuity when the switch is turned on. If there’s no continuity, you’ll need to replace the switch.

Make Sure the Ground Connection Is Secure

Locate the ground wire and ensure it’s attached to a clean, rust-free metal surface. Use a multimeter to confirm a good connection.

See a Mechanic for a Diagnosis

If the problem persists, it's best to consult a mechanic. They have the expertise and specialized equipment to diagnose the headlight system accurately.

Identify and Fix Common Headlight Problems

Scenario: Only One Headlight Is Not Working

1.Start by switching on the high beams—do both lights turn on?

2.A single burned-out bulb is the most common cause. Replace the bulb if necessary.

3.Some vehicles have separate fuses for the left and right headlamps. Before assuming the bulb is defective, use a test meter to check for power and ground at the light.

Scenario: Both Low Beams Are Not Working

1.Check if the high beams function correctly.

2.If the high beams work but the low beams do not, the dimmer switch may be faulty. Use a voltmeter to check for battery voltage at the bulb connectors while the lights are on—if voltage is present, the dimmer switch is working.

3.Although uncommon, both low-beam bulbs may have burned out. Use a test light at the headlamp connector—if the test light illuminates, replace the bulbs.

Scenario: Both High and Low Beams Are Not Working

1.Start by confirming power and ground at the headlamp connectors.

2.Inspect the headlamp circuit fuses to see if any have blown.

3.If the fuses are intact, check for power entering and exiting the headlamp switch.

4.Verify that the dimmer switch is functioning properly.

5.Inspect the headlamp relay.

6.Check for power at the relay and confirm that the relay coil control circuit is operational.

7.If the relay clicks when the headlamps are turned on, but there is no light output, test whether power is being sent from the relay to the headlamps—a relay can click without making proper electrical contact.

How to Replace a Headlight Bulb?

Disconnect the Battery: Prevent shocks by disconnecting the negative terminal.

Access the Bulb: Remove the headlight assembly (some models require removing the wheel well liner).

Unplug the Old Bulb: Twist the socket counterclockwise and pull it out.

Install the New Bulb: Avoid touching the glass. Secure it, reconnect the wiring, and test before reassembling.

Tips for Headlight Maintenance

To prevent future headlight issues, it’s important to perform regular inspections and maintenance:

•Clean and inspect headlights regularly. Built-up dirt and debris can damage headlights over time and prevent them from functioning properly. Wipe down headlights with a glass cleaner or plastic polish and inspect for any cracks or moisture buildup.

•Replace bulbs proactively. Don’t wait for bulbs to burn out before replacing them. Most bulbs degrade over time, producing less light. For more information, you can check this guide to see how long do headlight bulbs last.

•Consider LED headlight bulb upgrades. LED bulbs last much longer than incandescent bulbs and produce a brighter, whiter light. Upgrading to LED headlight bulbs means you won’t have to replace them for many years.

Don't know which headlight bulb size your vehicle takes? Put in your year, make and model and our automotive bulb finder will show you the right bulb for your ride.

Here are some common LED headlight bulb sizes we offer:

H11/H8/H9 LED headlight bulbs

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Common Questions about Headlights Not Turning On

Last Words

Fixing headlights should be a top priority for your safety on the road. For high-quality LED headlight bulbs you can rely on, SEALIGHT has got you covered. We provide free shipping, a generous 90-day return window, and lots of options - so you're in good hands.

With your headlights working like they should, you'll feel way more comfortable and safe when you're out and about, whatever the conditions are.