Diagnosing Problems With Your Car's Lights

Updated on March 21, 2019
Dan Ferrell profile image

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

Car lighting problems are often easy to fix.
Car lighting problems are often easy to fix. | Source

Car lighting problems can be caused by:

  • a bad bulb
  • corrosion
  • a loose wire
  • a bad ground
  • an electrical open or short
  • a failed switch or relay

And this is not only inconvenient. It can get you in trouble. For example, driving with one headlamp out, no indicator lights, or no brake lights is unsafe and can get you a fine.

However, most car lighting problems you'll face only require replacing a light bulb, cleaning some corrosion, or reattaching a wire.

Problems with a single light are usually caused by a burned-out bulb, while problems with two or more lights in a circuit are usually caused by electrical circuit problems.

In this guide, you'll find simple troubleshooting techniques for some of the most common light problems with headlights, turn signals, and general lights in a circuit.

For these tests, you'll need a 12V test light and in some cases a digital multimeter (DMM).

How to Use This Guide

The following sections deal with different types of light circuit problems. This will make it easier for you to concentrate on the particular light problem you may have.

If you need help locating or accessing parts and components in your vehicle, consult your vehicle repair manual. If you don't have the manual yet, you can get a relatively inexpensive Haynes manual from Amazon.

Haynes manuals come with step-by-step procedures for many maintenance, repair, and troubleshooting projects you can do at home.

If during your troubleshooting procedures you suspect an electrical open or a short circuit —for example, if you find no power is getting to a light bulb socket—check these other posts on how to find a car electrical open and how to find a car electrical short.

Index
I. One Low-Beam, High-Beam or Taillight Doesn't Work
II. Neither of My Low-Beams, High-Beams, or Taillights Are Working
III. My Turn Signal is Too Slow
IV. My Turn Signals Don't Work on One Side
Bonus: How to Test a Light Switch With an Ohmmeter
V. My Lights Flicker
VI. One of the Lights is Dim
VII. One of the Lights Doesn't Work
VIII. My Light Bulb Keeps Blowing Out
IX. All the Lights in the Circuit are Dim
X. All the Lights in the Circuit Have Failed

Handling Light Bulbs

When inspecting lights, don't touch the glass surface of lights bulbs that you know are good. Residual oil you may leave on the glass from your fingers and the bulb's operating temperature will shorten the bulb's service life.

Check light bulbs with an ohmmeter.
Check light bulbs with an ohmmeter. | Source

I. One Low-Beam, High-Beam, or Taillight Doesn't Work

When it comes to lights, headlights and taillights are a frequent cause of problems. Often, all you have to do is replace a bulb.

To troubleshoot the problem, just follow the next steps. The same switch controls both circuits, so they apply to both headlights and taillights.

You can use a 12V test light or digital multimeter for these tests. The advantage of using a DMM is that you'll know how much voltage a circuit is getting. The test light may not illuminate as bright when low voltage is present at the circuit.

NOTE: If you need a more detailed procedure than the one presented here, follow the steps in this other symptom-based diagnostic post to troubleshoot your headlights.

  1. Once you access the light bulb you need to check, take a look at the filaments.

    If one of the filaments is broken, replace the light bulb. If you can't tell if one of the filaments is broken, use your ohmmeter and follow the next steps:

    1. Set your ohmmeter to the lower setting or to continuity.
    2. Touch one lead to the ground pin (the one that connects to the black wire on the connector) on the bulb assembly, and the other lead to each of the other pins.
    3. If one of the tests results in infinite resistance or the ohmmeter doesn't beep, then one of the bulb filaments is broken and you need to replace the light bulb.
  2. Checking incoming voltage and ground

    Wiring problems are not common. However, circuit problems usually are the result of corrosion or a failing switch. So make sure to check for these issues.

    1. Unplug the electrical connector from the light bulb. Consult your vehicle repair manual, if you need help to unplug the connector.
    2. Set your meter to DC volts, and choose the 20 volts range, or auto-range.
    3. Turn on the headlights.
    4. Connect your black (ground) meter lead to battery ground or a good engine or chassis ground.
    5. Touch the meter power lead to each of the non-ground (black wire) connectors on the light electrical connector.

    Your meter should read 12V or battery voltage. If not, check for incoming and outgoing voltage at the headlight switch. If necessary, use the wiring diagram in your vehicle repair manual to trace for voltage along the circuit.

    1. Connect your black (ground) meter lead to the connector ground wire.
    2. Touch the meter power lead to the battery positive terminal.

    Your meter should read 12V. If not, trace the ground wire in the circuit. If necessary, use the wiring diagram in your vehicle repair manual to trace the ground.

    1. Turn off the headlights.

Dealing with xenon or HID headlights:

The easiest way to test an HID headlight is to swap the light bulb with the other light bulb. If the suspect bad bulb still fails, replace the light bulb. Otherwise, the problem could be with the ignitor or circuit.

Source

II. Neither of My Low-Beams, High-Beams, or Taillights Are Working

When both low-beams, high-beams or taillights fail, usually the relay for those lights has failed. However, a blown fuse, loose power or ground connection can also be the cause.

On some applications, the ignition switch may serve as a common connector for these lights as well. Consult the wiring diagram for your vehicle model, if necessary.

  1. First, check for a circuit blown fuse.

    Consult your car owner's manual or vehicle repair manual, if you need to locate the fuse for the circuit. The fuse box is usually located on the driver's or passenger's side under the dashboard, the glove compartment, or the engine bay.

  2. Checking incoming voltage and ground

    Gain access to the headlight switch wiring connections. If necessary, consult your vehicle repair manual to access the switch.

    1. Turn on the headlights.
    2. Connect your black (ground) meter lead to battery ground or to chassis ground.
    3. Touch the meter power lead to the incoming voltage (power wire) and outgoing voltage wire on the switch. Your meter should read 12V or battery voltage.

      • If there's not incoming voltage, trace the voltage to its source using your vehicle repair manual.
      • If you have incoming voltage but not outgoing voltage, either the switch is bad or the outgoing voltage wire is loose, corroded or bad.
    4. Trace the circuit ground, if necessary, using the wiring diagram in your vehicle repair manual.
    5. Turn off the headlights.

Modern car lighting systems come with lighting control modules (LCM) that can create particular problems that mimic electrical circuit problems like corrosion, loose or disconnected wires, and failed light bulbs. They may even prevent the headlights from going off or on. If your car model comes with a LCM and you haven't found the source of the problem, check the next video for some pointers about the light control module.

Source

III. My Turn Signal is Too Slow

NOTE: If you need a more detailed procedure to check the turn signals on your vehicle, check this other post Turn Signal Problems and Diagnosis.

Slow turn signal operation is not common but it can happen when corrosion, a loose wire, or a bad flasher unit affects the circuit.

  1. First, if you recently replaced one of the turn signal light bulbs, make sure you installed the correct bulb for your application.
  2. Gain access to the socket (remove the lens and light bulb) or wiring connections (behind the housing) on the slow signal.

    • Connect your voltmeter red lead to the power wire or connection in the light bulb socket (center contact), and your black meter lead to battery, engine or chassis ground.
    • Operate the turn signal you are testing and watch the meter. You should get 12 volts or battery voltage; otherwise, check for a loose wire, corrosion at the connection or at the switch.
  3. Repeat step two on the ground side of the connection, if necessary. But connect the meter red lead to battery power and meter black lead to socket ground (socket wall).

    You should get 12 volts or battery voltage; otherwise, make sure the socket is well connected to ground (usually chassis ground).

  4. If voltage checks OK in steps 2 and 3, you may have a faulty flasher unit.

A failed light bulb in the circuit can cause turn signal problems.
A failed light bulb in the circuit can cause turn signal problems. | Source

IV. My Turn Signals Don't Work on One Side

The most common reason for the turn signals to fail on one side is a burned-out bulb, so make sure to check the bulbs first.

  1. Gain access to the light bulbs on the turn signal side that doesn't work. If necessary, consult your vehicle repair manual.
  2. Remove and check each bulb.
  3. Usually, a burned-out bulb will show a dark side on the glass. Still, if you are unsure, you can check the bulb with the ohmmeter.

    • Select the lowest range, or continuity, in the Ohms scale of your multimeter.
    • Connect one of the meter leads to the connector on the light bulb, and the other lead to the outer casing on the bulb.
    • Your meter should beep or register some resistance; otherwise, the bulb is burned-out.
  4. Check the bulb's socket for proper voltage.

    • Select the 20-volt range on the "DC Voltage" scale of your meter.
    • Operate the turn signal and touch your meter's red lead to the socket's center terminal, and your meter's black lead to the metal side of the socket.
    • Your meter should register 12 volts or battery voltage; if not, a wire is loose or corrosion is affecting the circuit.
    • Check the power wire and ground connection at the socket. If necessary, trace the voltage at the power wire using your vehicle repair manual.

Source

Bonus: How to Test a Light Switch With an Ohmmeter

Sometimes it is possible to remove, or unplug, a switch from the vehicle and test it using an Ohmmeter.

  1. Unplug the electrical connector from the switch.
  2. If necessary, remove the switch from the vehicle for testing.
  3. Set your multimeter to a low range on the Ohms scale or to continuity.
  4. Touch one of the switch's terminals with one of the meter leads, and the other terminal with the other meter lead.

    • If the switch is normally open (lights are off until you operate the switch), there should be no continuity. Your meter should show infinite resistance or beep.
    • If the switch is normally closed (lights are on until you operate the switch, like the one for the doors), your meter should show some resistance or beep.
    • Results should be opposite when you operate the switch; otherwise, the switch is bad and you need to replace it.

If necessary, check the light bulb with an ohmmeter to verify its condition.
If necessary, check the light bulb with an ohmmeter to verify its condition. | Source

V. My Lights Flicker

Lights in a circuit may flash intermittently. The most common reason for a flickering light or lights is a loose wire or an electrical short.

  1. Using the wiring diagram, trace the wires in the circuit. You may find the diagram in the vehicle repair manual for your particular vehicle make and model.

    • If only one of the lights in the circuit flickers, check the electrical connection or socket for that light.
    • If all the lights in the circuit flicker, check the wiring diagram and inspect the common power or ground source for all the lights in the circuit like a switch, relay or fuse connections.
  2. Another potential problem source for flickering lights is a breaker that keeps cycling because of a short circuit. If you suspect an electrical short, this other post will help you find the short.

    • Check the wires in the circuit for damaged insulation or a loose wire.

VI. One of the Lights is Dim

Sometimes, a light that has turned dim or yellow only requires a new light bulb. Other times, a dim light can be a sign of a problem in the electrical circuit.

  1. Gain access to the light bulb and socket you want to check.
  2. Remove the light bulb and check for corrosion around the light bulb outer casing and the socket.
  3. Check the socket electrical connections for corrosion or loose wires.
  4. If necessary, check the socket and connections for voltage, following step 2 of the previous section.
  5. If proper voltage is reaching the socket and there's a good ground, replace the light bulb.

Light Switch Problems

Vehicles use different types of switches to operate lights in a circuit. Usually, a failed switch will either cause the lights to stay on all the time or keep the lights from coming on.

VII. One of the Lights Doesn't Work

Access the light bulb you want to check and make sure the connector is not loose. If there's a proper connection, then remove the light bulb from its socket.

  1. Check the bulb filament(s). If it's broken, replace the bulb. If you are not sure whether the bulb is good, continue with the next steps.

    • Set your ohmmeter to the lowest setting on the Ohms scale or continuity.
    • Touch one of the meter leads to the outer casing on the light bulb and the other meter lead to the connector at the bottom of the outer casing. If there's more than one connector at the bottom, test each one.
    • The meter should beep or register some resistance. If the readout displays infinite resistance, replace the bulb.
  2. Check the light bulb socket for corrosion. If necessary, check the socket for voltage using the following steps:

    • Set your voltmeter to 20 volts on the DC Volts scale.
    • Touch the meter red lead to the contact at the bottom of the socket, and the black (ground) lead to the sockets wall.
    • Turn on the switch for the circuit. Your meter should read about 12 volts or battery voltage. If it does and the bulb is good, the bulb is not sitting properly in the socket or the bulb terminals are worn; otherwise, continue.
    • Check the ground and power connections to the socket.
    • Test for incoming voltage at the power wire. Connect your meter black lead to a chassis, engine or battery ground, and the meter red lead to the socket power wire connection. If there's no voltage, check the wire path between the socket connection and the point where it connects to the rest of the circuit.
    • Test for ground. Connect your meter red lead to battery power (+) and the meter black lead to the socket ground connection. This is either chassis ground or a wire that provides ground through a connector. Your meter should display about 12 volts, that is, battery voltage; if it doesn't, fix the ground connection to the socket.

VIII. My Light Bulb Keeps Blowing Out

A light bulb that keeps blowing out is not common. This type of problem is usually caused by water in the assembly or an intermittent electrical short.

Take a close look inside the assembly. If there's water at the bottom of the light housing, this might be your problem. Water into the assembly is being filtered through a failed seal or a crack in the housing. A quick and simple repair is to drill a small hole at the bottom of the housing to allow water to drain out. Of course, you can always replace the assembly.

Another reason for a bulb that keeps blowing out is an intermittent electrical short. If you suspect there's a short in the circuit, this other post on finding a car electrical short can help you here.

Taillights are controlled through the headlamps switch.
Taillights are controlled through the headlamps switch. | Source

IX. All the Lights in the Circuit are Dim

This problem is similar to that affecting a single light in the circuit (see the previous section), except the problem affects two or more lights in a circuit. Still, it is possible the lights bulbs are worn and need to be replaced. But this can also indicate a problem with the charging system.

If the lights seem dim when the engine is idling but become bright with an increase in engine speed (rpm), check the charging system for problems first. Then, if necessary, continue with the next steps.

  1. Measure incoming and outgoing voltage at the protective device for the circuit (fuse or breaker).
  2. Measure incoming and outgoing voltage at the control device or switch for the circuit.
  3. Measure voltage at the point in the path where power is distributed to the whole circuit, if necessary.

    • Connect your meter black lead to chassis, engine or battery ground, and the meter red lead to each point in the circuit you need to test along the power path of the circuit.
    • Voltage should be around 12 volts or battery voltage. Any test point that shows less than 12 volts is where the problem is located. Check for corrosion or loose wires.

If necessary, check chassis and engine grounds. This other post on automotive voltage drop test can help you check grounds and the power side of the circuit.

Source

X. All the Lights in the Circuit Have Failed

When all the lights in a given circuit have failed, usually the fix is relatively easy. You need to get to the points more likely to fail and that can affect power feed to the lights.

  1. First, check the circuit fuse or breaker.
  2. Check for incoming and outgoing voltage at the control switch for the circuit. You may need to remove parts or components to gain access to the switch connector or connections. You can use a test light to check for the presence of voltage at the switch.
    • Connect your test light to chassis, battery or engine ground.
    • Turn on the switch, and the ignition switch to On if necessary, to activate the circuit.
    • Probe the power wire for incoming voltage at the switch. If the switch has a connector, make sure the wire coming from power source has voltage, and the switch's wire is allowing voltage through. If not, check the connector for corrosion or a loose wire.
    • If there's not incoming voltage, check for the presence of voltage at both sides of the fuse or protecting device. If there's no voltage at the fuse, the problem lies between the fuse and the battery. If there's power at the fuse and voltage is going to the circuit from the fuse, the problem lies in the path between the fuse and the switch.
    • Check that voltage is going from the switch to the lights; otherwise, either the switch is bad or the wire connecting from the switch to the lights in the circuit is loose or corroded. Check the connector. If necessary, check the circuit path between the switch and the point where the wire connects to the rest of the lights.

To locate parts, access components and identify wires in the circuit, consult the wiring diagram in your vehicle repair manual for your particular model.

Consult the wiring diagram for your car lights to diagnose hard to fix light problems.
Consult the wiring diagram for your car lights to diagnose hard to fix light problems. | Source

Car lighting problems are common and often easy to fix. Usually, all you need is a test light, a multimeter, and a Phillips screwdriver. Still, there will be times when you need to trace the fault through a circuit, but even then the fault is usually not hard to find. This guide will help you locate the fault and fix your lights in a short time.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

    © 2019 Dan Ferrell

    Comments

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      • Dan Ferrell profile imageAUTHOR

        Dan Ferrell 

        7 weeks ago

        Check the light bulbs and the sockets. There could be a filament broken or a loose or faulty connection (power or ground). Hope it helps.

      • profile image

        Johndesouza 

        7 weeks ago

        Hi, When I turn on my head lights, the indicator flasher, do not flash. They are still. This is ok when the headlights are off. Ford explorer 2005. I replaced the flasher relay, but problem not solved.is the ground of the flasher lights or the headlights bad? Please advice

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