How to diagnose electrical headlight problems on your car

Written by matt scheer
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If the headlights on your car are not working, one of four major electrical components may have failed: the headlight bulbs themselves, the wiring harness bringing power to the headlights, the fuses that operate on the circuit to protect it from power surges, or the headlight switch, which allows the driver to change between low and high beams. Any one of these components may be the source of your headlight problems. Diagnosing the issue is a straightforward process.

Skill level:
Moderate

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Things you need

  • Voltmeter

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Instructions

  1. 1

    Turn on your headlights. Replace whichever headlight bulbs do not turn on. Although the process for replacing a headlight bulb varies by make and model of vehicle, in general, to remove the bulb: disconnect the wiring harness from the rear of the headlight assembly, disengage the retaining clip and pull out the bulb. Insert the replacement bulb, secure it with the retaining clip, and plug back in the wiring harness. Test the headlights again. Continue to the next step if any do not turn on.

  2. 2

    Open the engine compartment fuse box. Pull the fuse that operates on the non-functioning headlight circuit. Inspect the fuse for discolouration. Replace as needed. Turn the headlights on again. Continue to the next step if any still do not turn on.

  3. 3

    Connect the negative lead of the voltmeter to the negative terminal on the car battery. Connect the positive lead of the voltmeter to the positive lead of the wiring harness. The positive lead has power coming through it and is usually the red wire. Verify that power is coming through the wiring harness. Replace it if no power is coming through. Turn the headlights on again. Continue to the next step if they still do not turn on.

  4. 4

    Disconnect the negative battery cable from the car battery. Remove the bezel around the steering column of the steering wheel. Unplug the wiring block from the back of the headlight switch. Check whether the contacts on the wiring block or headlight switch are discoloured. Clean them gently with a pencil eraser. Plug back in and turn the headlights on again. If they still do not work, you may need to replace the headlight switch.

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