A headlight that won’t turn on usually points to a bad bulb, blown fuse, failed relay, switch fault, wiring break, or a control module issue.
Your headlight failed and darkness hit back. Don’t panic. You can trace the fault with a few clear checks and a basic tool kit. This guide gives you fast steps, a smart table, and repair paths that work on halogen, HID, and LED setups. Read once, fix once, drive away with light.
Headlight not turning on: fast checks
Start with the basics. Many outages come down to a simple part swap or a quick clean. Work from easy to advanced so you don’t chase ghosts.
| Symptom | Likely cause | Quick test |
|---|---|---|
| Both low beams out | Blown fuse, bad relay, failed switch, BCM fault | Check headlight fuse and relay, then test switch output |
| One side out | Burned bulb, corroded socket, local ground break | Swap bulbs side to side; inspect socket and ground |
| High beam works, low beam dead | Low-beam fuse, relay, or driver | Probe low-beam feed at connector with a test light |
| Lights flicker | Loose plug, weak ground, failing ballast/driver | Wiggle harness; check voltage drop to ground |
| Lights work only on “auto” or only on “manual” | Ambient light sensor or switch logic fault | Toggle modes; scan for codes if equipped |
| Headlight won’t stay on after bumps | Broken filament, loose relay, poor contact | Tap housing and relay box; re-seat parts |
| Moisture in lens, dim output | Seal leak, oxidized contacts | Dry the housing; clean and seal |
Safety and prep
Park on level ground, set the parking brake, and switch the lights off. Wear gloves so oils don’t bake onto a bulb. Keep a multimeter or a test light handy. If you work near a live feed, pull the battery negative cable once you finish tests that need power.
Step-by-step troubleshooting
Check the headlight switch and dimmer
Spin the switch through off, parking, and headlight. Flip the stalk for high and low. If the cluster backlight changes and the headlamps stay dark, the switch may not pass current to the low-beam feed. On many cars the stalk routes ground to a relay coil. No ground, no click, no light. If you hear no relay click at all, move to the fuse box next.
Inspect the bulb or LED module
Halogen bulbs fail more than any other part. Pull the cap, twist the holder, and check the filament. A gray haze or a broken filament means the bulb is done. On LED units, look for melted plastic, a loose heat sink, or a failed driver pack. Swap the suspect bulb to the other side when the design allows. If the fault follows the bulb, you found it.
Test the headlight fuse
Open the under-hood or cabin fuse panel. Find the headlight or low-beam slot. Use a test light across the tiny probe holes on top of the fuse. Power on both sides means the fuse is good. Power on one side only means the fuse is blown. Replace with the same rating. If it pops again, stop and hunt for a short before you melt a harness.
Swap or test the headlight relay
Relays switch high current with a small coil signal. If the coil never gets power or ground from the switch or body module, the lights stay dark. Many panels use twin relays with the same part number. Swap a known good twin to test. If the lights wake up, buy a new relay and call it done.
Check ground points
A weak ground can mimic a dead bulb. Follow the harness to a fender or radiator panel stud. Clean rust, add dielectric grease, and tighten the fastener. Measure voltage drop from the bulb ground to the battery negative while the light is commanded on. Anything above 0.3 V hints at a ground issue.
Measure power at the connector
Back-probe the low-beam feed with the lights on. You want battery voltage under load. If you read low volts or none, trace back to the relay and the switch feed. If you read full voltage and the bulb still stays dark, the bulb or module is bad.
Look at the multifunction switch
Worn contacts inside the stalk can drop the low-beam circuit. If jiggling the stalk brings the lights to life, the switch is suspect. Many cars let you pull the column shroud and test continuity across the low-beam pins. Replace the switch if readings drop out while you move the lever.
Check DRL or auto-light sensors
Daytime running light modules and photo sensors can confuse the headlight logic. Cover the sensor on the dash and see if the lamps switch modes. If DRL works but full low beam won’t, the body module may not be sending the right command to the relay. Scan tools can read codes and live data on many late models.
HID and LED extras
HID systems add a ballast; LEDs add a driver. Either can fail and leave you with a black lens. Listen for a faint buzz from an HID ballast on power-up. No sound and no arc points to the ballast. On LED kits, poor heat management cooks drivers. If your car shipped with halogen, stick with approved lamps or a full housing swap that meets beam specs.
When both sides fail
Two bulbs rarely die in the same minute. When both sides drop, think power feed, ground bus, relay control, or a body control module (BCM). Check the main headlight feed fuse first. Then verify the relay coil gets a command. If the coil has power and ground yet no output, replace the relay. If the coil never gets a command, the switch or BCM needs testing.
Fixes, costs, and time
Parts prices swing by design, but the time often stays short. Most fixes land under an hour for a driveway mechanic. Here’s what you can expect in broad strokes.
- Halogen bulb: 10–20 minutes per side.
- LED module in a sealed unit: 30–90 minutes.
- Relay or fuse: under 10 minutes.
- Multifunction switch: 30–120 minutes.
- Ground repair or connector clean-up: 20–45 minutes.
- BCM diagnosis: shop scan and wiring checks add time.
For legal and safety context, the U.S. lighting rule that covers headlamps is FMVSS No. 108. For practical wiring checks and part names, this AutoZone troubleshooting guide walks through fuses, relays, and bulbs step by step. Bookmark both so you have the rule and the fix on hand during the job. Keep a copy in the glove box for quick roadside checks.
Aiming and after-repair checks
Once the light comes back, aim the beams. A low aim wastes light; a high aim adds glare. Park 25 feet from a wall on level ground. Set tire pressures, remove heavy cargo, and bounce the car to settle the springs. Mark the center of each lens on the wall with tape. Use the adjusters to set the cutoff lines at or just below the lens center height. Take a short drive and fine-tune.
Moisture, heat, and corrosion control
Water inside a housing kills bulbs and drivers. Pull the vent caps, dry the unit with gentle heat, and reseal. Replace torn backshell caps. Use dielectric grease on the connector to slow future corrosion. If the lens is cloudy, clean and coat it so the new bulb gets a fair shot.
Tools and readings that matter
A test light is fast and simple. A multimeter helps with drop tests. A scan tool adds data when a module runs the show. The table below turns readings into calls you can act on.
| Test point | Target reading | Bad reading means |
|---|---|---|
| Bulb connector, low-beam feed to ground | Battery voltage with switch on | No volts: upstream fault; low volts: high resistance |
| Relay coil control | Battery voltage or ground, as designed | No control: bad switch or BCM path |
| Ground drop (bulb ground to battery-) | ≤ 0.3 V under load | Higher drop: clean or repair ground |
| Fuse test tabs | Power on both tabs with lights on | Power on one tab: blown fuse |
| HID ballast input | Battery voltage at power-up | No power: relay or harness issue |
| LED driver input | Battery voltage steady | Pulse or drop: weak connection or driver |
Prevention tips that save night drives
Keep spare fuses in the glove box. Clean lens covers twice a year. Replace bulbs in pairs so color and brightness match. Secure loose items in the engine bay that could rub a harness. After deep washes or rain, pop the hood and check for moisture near the back of the lamps. A minute now beats a roadside stop later.
When to book a shop
Book a pro when a fuse blows again right away, when both sides die with a good fuse and relay, or when a scan shows BCM faults. Warranty and recall work belongs at the dealer. If wiring damage is clear or the car has smart lighting with coded parts, a shop visit saves time.
Final check before you roll
Turn the lights on, set low beam, then high beam. Walk around the car. Check the dash beam icon, side markers, and plate lamps. Wiggle the stalk, the switch, and the connector to catch loose spots now, not on the road. Set the aim, pack a spare fuse, and you’re set for the next night drive. Confirm fog lights and DRL work as designed too.