When Your Headlights Are Not Working: Diagnosis Based on Symptoms

Updated on October 30, 2018
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Headlights come in many configurations but they all share the same basic principles of operation.
Headlights come in many configurations but they all share the same basic principles of operation. | Source

If your headlights aren't working, there may be one or more causes:

  • Burned-out bulb
  • Corroded socket
  • Blown-out fuse
  • Electrical short
  • Electrical open
  • Charging system problems

Sometimes, all you'll need to do is replace a light bulb; other times, you need to check a socket or test the circuit and switches.

Although some of these tasks may seem complicated, most of the time you'll be dealing with simple issues. Perhaps your most difficult task will be getting access to components, depending on your particular model.

However, to make a successful repair and avoid chasing false leads, you need to know where to start and what potential causes you may be dealing with.

This guide shows you the causes behind some of the most common headlight failures and some practical techniques that can help you diagnose and repair your headlights faster.

The following sections are divided into the most common headlight symptoms. Go to the one that seems to deal with your problem, but keep in mind that information included in the other sections may also help in dealing with your particular issue.

Index
I. One of the Headlamps Doesn't Work
II. My Headlights Flicker
III. My Headlights Are Dim
IV. My Headlights Don't Work
V. Finding an Open in the Circuit
VI. My Headlamp Fuse Keeps Blowing
VII. My Car Uses HID Headlamps
VIII. Fixing Headlight Problems Faster
Check the light bulbs when troubleshooting headlight problems.
Check the light bulbs when troubleshooting headlight problems. | Source

I. One of the Headlamps Doesn't Work

When one headlamp doesn't work, whether it's the high or low beam, check the lamp or bulb itself first. Getting access to a headlamp or bulb for inspection or replacement is not too difficult.

Removing a Headlamp

Older vehicle models use a sealed-beam headlamp, usually secured by a bezel and a retaining ring using a few screws. Look around the perimeter of the headlamp and remove the screws that hold the lamp in place.

If necessary, access the electrical connector from the rear of the assembly.

Sealed-beam headlamps need to be replaced as a unit when they fail.
Sealed-beam headlamps need to be replaced as a unit when they fail. | Source

Removing a Halogen Bulb

Most modern vehicles use "composite headlights, that is, a halogen bulb inside the headlamp housing. You access the bulb through the rear of the light assembly.

  1. Pop the hood open.
  2. Locate the rear of the headlight lamp you want to replace.
  3. To unplug the bulb's electrical connector:
    • Push and twist the base of the connector about a quarter of a turn counterclockwise.
    • Then, carefully pull the socket out of the housing, or
    • Unscrew the ring that holds the connector in place about a quarter of a turn counterclockwise.
    • Then, remove the ring and pull the socket out of the housing.
    These are the most common methods of securing a connector to the bulb's socket. But consult your vehicle repair manual to prevent damage to the assembly, if necessary.

When inspecting the bulb, look at the filaments inside. If the bulb is not good, its filament(s) will be broken in half.

Checking Continuity

If you are not sure the filament is broken, you can check for continuity:

  1. Set your digital voltmeter (DMM) to the lowest setting on the Resistance scale.
  2. Turn on your DMM and touch the meter probes on each side of the bulb contacts.

If there's continuity, your meter will beep or show zero resistance; otherwise, the display will show infinite resistance, overload, or no sound, depending on your particular meter configuration.

The next video shows you how a blown fuse can affect one headlight.

Caution: Handling Light Bulbs

When handling a light bulb, never touch the surface of the bulb. The oil from the skin on your fingers, combined with the high working temperatures of the bulb, will reduce the bulb's service life or cause it to shatter soon.

II. My Headlights Flicker

Although not too common, flickering headlights can also become a problem. They may be caused by:

  • A bad headlamp switch
  • Unusual resistance in the circuit
  • A loose or corroded connection
  • Poor circuit grounding
  • A bad multifunction switch

First, check whether both headlamps are flickering, or only one. If only one of the lamps is flickering, concentrate on that part of the circuit:

  1. Check the power and ground wires that connect to the headlamp.
  2. Check the electrical connector.
  3. Inspect the bulb socket for corrosion or a loose wire on the connector.

If both headlamps are flickering, check the part of the circuit that controls both lamps:

  1. Flip the switch to low- and high-beam, and see if the lights flicker in both modes.
  2. Check the switch.
  3. Check for loose wires at the switch.
  4. Check the multifunction switch.
  5. Check the power side of the circuit for a loose wire or connector.

You may need the diagram for your vehicle model to trace splices and connectors.

Wear and electrical problems can cause a light to dim.
Wear and electrical problems can cause a light to dim. | Source

III. My Headlights are Dim

Several faults may cause headlights to be dim:

  • corrosion or bad connections in the power circuit
  • headlamps or bulbs worn out
  • damaged wires in the circuit, or
  • low charging system output.

Make sure the headlight lenses are not foggy. Light bulbs, also, may also be worn through years of service.

You may notice that your headlights dim in a particular engine operating condition.

If the headlights are dim with the engine off or while idling:

  • Verify that battery terminals are clean and tight.
  • Make sure your battery is charged or in good condition. You may need to test the battery.
  • Check charging system output and drive belt for proper operation.
  • Check the light bulbs for proper installation or wear.

If the headlights are dim when the engine speed is above idle:

  • Check the charging system output and the drive belt for proper operation.
  • Check for voltage drop in the headlamp circuit.
  • Check the light bulbs.

Checking for Voltage Drops

A common cause of bad headlight operation is unwanted resistance in the circuit. Because unwanted resistance causes voltage drops, a practical way to find high resistance in an electrical circuit is to use a digital voltmeter to check for voltage drops.

The most common causes of high voltage drops are:

  • corrosion buildup in electrical connectors,
  • carbon buildup around burned contacts in relays, and
  • loose or damaged wires.

Although the check itself is simple, getting access to switches, relays, and connectors to test may be difficult on some models. Consult your repair manual.

Checking for Voltage Drop on the Positive Side

  1. Make sure you have good access to the right or left side of the headlamp you are going to be testing. You may need to pull out the socket and bulb assembly while conducting this test. If you are testing a halogen-bulb type lamp, don't touch the light bulb or you'll leave oil residue on its surface that can affect the bulb's performance.

  2. Set your DMM to the lower setting on the DC voltage scale.

  3. Connect the meter's red probe to the positive battery post, and the meter's black probe to the power terminal wire on the socket.

  4. Turn on the headlights.

  5. Turn on the DMM.

Ideally, you should get about 200 mV of voltage drop. Some manufacturers consider a voltage drop of 0.5 V (500 mV) reasonable. Consult your vehicle repair manual, if necessary.

If the voltage drop seems high, check voltage drop across the battery post and terminal, connectors, relays and switches on that side of the circuit.

You may need the diagram for your particular vehicle make and model. You'll find this diagram in the repair manual for your car. If you don't have this manual, you can get a relatively inexpensive Haynes repair manual from Amazon online.

How to Check Voltage Drop on the Ground Side of the Circuit:

  1. Connect the black meter probe to the negative post on the battery, and the red meter probe to the ground terminal on the light socket.

  2. Turn on the circuit.

  3. Turn on your DMM.

You should get about 100 mV of voltage drop.

If necessary, check for voltage drop between the negative battery post and terminal, and verify that the ground body connection from the battery is tight and free of corrosion. Test it for voltage drop, if necessary.

How to Check for Voltage drop on a Switch, Connector, or Relay in the Headlamp Circuit:

  1. Connect your voltmeter across the input and output voltage terminals of the switch, splice or relay.

  2. Turn on the circuit.

  3. Compare your DMM readings with the values in the table below.

If necessary, consult your vehicle repair manual to gain access to components and locate and identify wires.

Common Voltage Drops

Element
Voltage Drop
Wire or connector
200 mV
Switch or relay
300 mV
Ground connection
100 mV
You can use these values as a general guide when measuring voltage drop across wires, switches and grounds.
A failed relay can also cause the headlights to fail.
A failed relay can also cause the headlights to fail. | Source

IV. My Headlights Don't Work

Sometimes, none of the headlights may work, in either mode. Before you start troubleshooting wires and components, verify that the circuit fuse(s) is good and properly connected.

Also, even if both of your headlights don't work, you want to check the light bulbs and make sure they are in good condition, and not burned out.

Checking for Electrical Faults

Several electrical faults can keep the headlights from working:

  • Blown fuse
  • No power to the circuit
  • Bad ground
  • Failed switch
  • Bad multifunction switch
  • Bad dimmer switch
  • Open in the circuit

If just the low or high beams don't work:

  • Check the relay for that mode.
  • Check the switch for that mode.

You may need to consult your vehicle repair manual to locate the relay you need to test.

How to Check That Power is Getting to the Headlights: Use a Test Light

  1. Unplug the light bulb electrical connector.
  2. Turn on the headlights.
  3. Connect your test light to a good ground and touch the test light probe to the power side of the connector terminal.

If the test light glows, current is getting to the headlamp. Check the socket and connector for damage, corrosion or a loose wire.

If the test light doesn't glow, check the circuit for an electrical open. Head to the following section Finding an Open in the Circuit below.

To Check the Ground Side of the Circuit:

  1. Connect the test light to battery power.
  2. Touch the test light probe to the socket's ground on the connector terminal.

    • If the test light glows, check the bulb socket and connector for corrosion or a bad contact.
    • If the test light doesn't glow, check the ground side of the circuit for an open. See the following procedure.

Some vehicle models come equipped with special headlight modules. Consult your vehicle repair manual for any tests you may be able to do in case a problem develops with one of these components.

Check the wiring diagram for your model to identify wires and trace electrical shorts and opens.
Check the wiring diagram for your model to identify wires and trace electrical shorts and opens. | Source

V. Finding an "Open" in the Circuit

An "open" in the headlights circuit can keep one or all the headlamps from working. The open can be caused by a disconnected wire, a loose electrical connection, corrosion in a connector, or a failed switch or relay.

Check the fuse:

  1. Using a test light, check for incoming power at the circuit fuse(s). If there's no power there, check the connections to the panel, and the condition of the wire and connections between the panel and the battery.
  2. Check for power on the output side of the fuse. If there's no power there, check the fuse and the panel connections.

Check the light socket:

  1. Verify power is reaching the socket.
  2. Make sure you have a good ground at the socket as well.
  3. If there's no power reaching the socket, but there is power from the fuse panel into the circuit, check the circuit between those two points.You'll need to check power going to the switch and relay, depending on your particular model. If necessary, consult your vehicle repair manual to gain access to components.

Check the headlight switch:

  1. Verify that power is reaching the switch.
  2. Check that power is going from the switch to the circuit.

    • If there's no power going to the headlight switch, check the wire between the switch and the fuse panel.
    • If there's no power going from the switch to the rest of the circuit, check for voltage drop at the switch (See Checking voltage drop, in III. My Headlights are Dim). If voltage drop is high, replace the switch.
    • If power is going from the switch to the headlights, check that part of the circuit between the switch and the headlamps. Check your diagram for splices and connectors in that part of the circuit that might have been disconnected or corroded.

Consult the electrical diagram for your particular vehicle make and model to identify wires and connectors in the circuit.

Check the circuit fuses.
Check the circuit fuses. | Source

VI. My Headlamp Fuse Keeps Blowing

Trying to find the source of an electrical short in a circuit can be tricky at times.

A short happens when a power or hot wire (the one carrying current to the loads in the circuit) finds a short path to a grounded surface. Most likely, the short will cause the circuit fuse to blow or one or more light bulbs to burn out.

Shorts can happen when:

  1. A hot wire's insulation breaks and the wire touches the body metal.
  2. A hot wire becomes pinched between two parts, cutting through the insulation and making ground.
  3. A hot wire disconnects from the component and hangs loose, touching ground.
  4. A hot surface melts a hot wire's insulation, allowing the bare wire to touch a grounded surface.

This common way to locate shorts may work for you:

  1. Connect a test light across the blown fuse or circuit breaker. If the test light glows, there's a short in the circuit.
  2. Disconnect the light socket, relay, and switch, one at a time.
  3. When the test light stops glowing, you've found the part of the circuit where the short circuit is located.

If necessary, consult your vehicle repair manual to locate components and identify wires.

A modern HID headlight.
A modern HID headlight. | Source

VII. My Car Uses HID Headlamps

Many modern vehicles come equipped with high-intensity discharge (HID) or xenon headlamps. You can tell by the bluish-white light they produce.

This type of lights are electronically controlled and don't have filaments as regular light bulbs do. Instead, an electric arc between two electrodes is directed at a gas (usually xenon) inside the bulb to produce the light.

To test the light bulb:

A common procedure you can do at home is to swap the headlamp bulbs between the two front assemblies. If after the swap the "failed" headlamp works and the other one doesn't, you need to replace the bulb.

To test the igniter:

If your model has an igniter separate from the light assembly, you can do the same test on the igniters to make sure you are not dealing with a failed component.

HID light bulbs require over 15,000 volts to bridge the electrodes' gap. After that the voltage drops below 100 volts.

If you are having problems with the circuit or one of its components, consult your vehicle repair manual first before conducting any tests.

Werther you have old style lamps or a more modern design, follow the logical steps to make your lights shine again.
Werther you have old style lamps or a more modern design, follow the logical steps to make your lights shine again. | Source

VIII. Fixing Headlight Problems Faster

Using a symptoms-based diagnostic to check headlight problems can speed up your repair.

But remember that although lighting systems have to stick to a basic setup demanded by federal laws, you'll still find different configurations, depending on your car's make and model. So, always check your vehicle repair manual to properly identify wires, components and mode of operation when necessary.

With the use of the simple strategies described in this guide, you can fix your headlights much faster at home, using a few simple tools, and save on repairs,

Questions & Answers

    © 2018 Dan Ferrell

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