When low-beam headlights do not work, but high beams are functioning correctly, the most likely cause is a bad dimmer switch. A bad dimmer switch can also result in the opposite problem, where low beams working but high beams are not. Dimmer switches are usually integrated in the steering column where the headlight switch is mounted.
To test the dimmer switch, necessitates removing part of the steering wheel column and possibly the dash panels in order to access the wires. A multimeter is necessary for testing voltage, and a wire-piercing probe makes the test process easier. There are three relevant circuits; the high-beam headlight feed, the low-beam headlight feed and the headlight-switch input. Check the headlight-switch input first to make sure a signal is present. Test the low-beam feed next and determine whether any voltage is present in the feed. If, after several tests, the multimeter does not register voltage, the low-beam switch is faulty.
Repeat the testing with the high-beam headlight switch only if the low-beam switch is confirmed to be working. If, after several tests, the multimeter does not register voltage, then the low-high-beam dimmer switch is faulty.
Headlight bulbs contain both high-beam and low-beam filaments. If the dimmer switch is not at fault, one of the filaments may have gone bad, and the bulb must be replaced.