Malfunctioning low-beam headlights are often caused by broken dimmer switches, burned-out light bulbs or broken running-light relay switches. If the problem is electrical, it is important to take the vehicle to a qualified mechanic for diagnosis and repair.
In some cases, there is nothing wrong with the low-beam headlights other than the fact that the headlight covers have become cloudy or dirty, preventing light from escaping. Cleaning the covers with toothpaste usually eliminates this problem.
The dimmer switch on a vehicle is located on the steering column, dashboard or foot board. When the switch no longer makes the correct connection with the car's electrical system, the low beams will not activate. Consequently, the high beams might work, but the low beams do not.
A burned-out light bulb could also cause low-beam lights to malfunction. However, this usually means that the headlights will not work regardless of the dimmer switch setting. Turning on the headlights while facing a blank wall is the easiest way to tell if the bulbs are unable to work at any setting.
Relay switches are also common reasons for malfunctioning headlights. The magnetic switch that, in modern vehicles, is controlled by the car's computer system. If the running light relay malfunctions, the low-beam headlights will not work.