Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairMedium
A vehicle owner reports that their high-beam headlights work correctly, but the low-beam headlights do not illuminate on either side. All other exterior lights are functioning normally. The vehicle uses a dual-filament H4 type bulb for the headlights. What is the MOST likely cause?
- AAn open circuit in the common ground wire for both low beams.
- BA faulty headlight switch.
- CA defective high-beam relay.
- DBoth low-beam filaments in the H4 bulbs are burnt out.
Show answer & explanationAnswer & explanation
Correct answer: D. Both low-beam filaments in the H4 bulbs are burnt out.
H4 bulbs have two filaments, one for high and one for low beam. If only the low beams are out on both sides, the most common issue is that both low-beam filaments have failed. A common ground or power supply issue would typically affect both high and low beams, or one side entirely.
Why the other options are wrong
- A. An open common ground wire for both low beams would prevent both from working, but H4 bulbs often share a common ground for both filaments, meaning high beams would also be affected if the common ground was open.
- B. A faulty headlight switch could cause both low beams to fail, but it's less common than bulb failure and often affects other functions controlled by the switch.
- C. A defective high-beam relay would prevent the high beams from working, not the low beams.
Dual-Filament Bulb Failure
Dual-filament bulbs contain two separate filaments for different lighting functions (e.g., high/low beam, park/turn). One filament can fail while the other continues to operate, leading to partial lighting functionality.
- Common in older headlights (H4) and combination park/turn signals (1157).
- Failure of one filament does not necessarily affect the other.
- Diagnosis involves checking continuity of each filament and voltage at socket.
Memory trick: One filament down, the other still shines around!