Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairMedium
A vehicle's high-beam headlights function, but the low-beam headlights do not. When the headlight switch is in the low-beam position, a test light connected to the low-beam wire at the headlight connector illuminates brightly. The ground connection for the headlights is verified to be good. What is the most likely cause?
- ABurnt-out low-beam filaments in both headlight bulbs
- BA faulty headlight switch
- CA short to ground in the low-beam circuit
- DAn open circuit in the low-beam wiring
Show answer & explanationAnswer & explanation
Correct answer: A. Burnt-out low-beam filaments in both headlight bulbs
If the test light illuminates brightly, it indicates that power is successfully reaching the headlight connector. Since the high beams (which often use a different filament in the same bulb or a separate bulb) work, and ground is good, the most probable cause for both low beams not working is that their specific filaments in both bulbs have burned out.
Why the other options are wrong
- B. A faulty headlight switch would likely prevent power from reaching the low-beam wire, meaning the test light would not illuminate.
- C. A short to ground would typically blow a fuse or prevent power from being present at the connector, or cause other electrical issues, which is contradicted by the test light illuminating brightly.
- D. An open circuit in the wiring would prevent power from reaching the connector, so the test light would not illuminate.
Dual-Filament Bulb Failure
Headlight bulbs often contain separate filaments for high and low beams, allowing one to fail while the other still functions.
- Common for both low beams to fail around the same time.
- High beam functionality confirms power to the assembly.
- Test light confirms power at the connector.
Memory trick: High's bright, low's plight, bulbs might!