In this guide, you will learn how to diagnose headlight problems. For example, cases where you have replaced the light bulb, but one of the headlights is not working.  

Most headlight failures are caused by a blown fuse. Other common problems include bad relay, switch or module. Wiring problems, especially inside the headlight, can also cause a headlight to stop working.  

Symptoms

  • One headlight don't work
  • Highlight not working bulb ok
  • One headlight not working on low beam
  • Headlight not working after replacing the bulb

Possible Causes

  • Blow light bulb
  • Blown fuse
  • Bad relay
  • Damaged wiring harness 
  • Faulty light switch
  • Defective light module

Troubleshooting Steps

Check Fuses

hyundai low beam problems

If one or both headlights don't work after you replace the bulbs, check the fuses. 

The low beams and high beams have their designated fuses for each of the circuits. In some car models, you will have a separate fuse for the left and right low beams.

nissan low beam problems

Locate the fuse box under the engine and by the dashboard. Find the fuse for the low beam and for the high beam.

If your lights are working on a high beam but not on a low beam, check the fuses first. Fuses are very inexpensive and easy to replace. If you find a burned out fuse order a box of 120 assorted fuses online and replace the burned fuse with the same color, which should also be the same ampere rating.

Check Headlamp Wiring

passat low beam problem

Carefully inspect the contact at the terminals at the light bulb and also the wiring inside the headlight. In the picture above you can see how the insulation had melted.

damaged headlight wiring

In this picture you can see how damaged wires inside the headlight had to be repaired. 

Light Switch

bad headlight switch

Another possible issue could be a faulty headlight light switch.

The light switch on the dash turns on the light, but switching between low and high beams is performed with the lever on the steering wheel column. This is known as the multi-function combination switch which is typically integrated with the turn signal switch.

To replace the multi-function combination switch it is required that the steering wheel is removed. This is a Do-It-Yourself project and should take you no more than two hours. An experienced mechanic should be able to replace the switch in about one hour.

Lights Relay

low beam headlight problem

In some models, the problem could also be the low or high beam relay. The relays are located in the fuse box in the engine compartment.

Replacement automotive relays can be purchased online and can be easily replaced by any car owner. 

Conclusion

If you have the circuit diagram and a digital multimeter you can do further troubleshooting to diagnose this problem. A few things that you should check is to find out if the light switch is receiving power. Check if power is being transmitted to the multi-function combination switch.

Switch between the high and the low beam and see if there is continuity at the terminals of the light and the combination switch. You cannot perform much troubleshooting without a multimeter so start by getting a Digital Multimeter.

If your headlights are not working,  you might not be able to see the road clearly. Properly working headlights are critical for your safety.