Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairMedium
A technician is diagnosing a complaint that the vehicle's left taillight and left front parking light are both inoperative. The left turn signal and brake light functions on the same side work correctly. The fuses are all good. What is the MOST likely cause?
- AAn open circuit in the common ground wire shared by the left taillight and left front parking light.
- BA faulty left turn signal/parking light bulb (1157 type) in the front.
- CA defective headlight switch or parking light relay.
- DA break in the power wire specifically for the left parking/taillight circuit.
Show answer & explanationAnswer & explanation
Correct answer: D. A break in the power wire specifically for the left parking/taillight circuit.
Since both the left taillight (rear) and left front parking light are out, but turn signals and brake lights work, it indicates a problem specific to the parking light circuit on the left side. An open power wire for this specific circuit would prevent both lights from illuminating.
Why the other options are wrong
- A. If the common ground for both was open, it would likely affect the turn signal and brake light functions as well, which are reported as working.
- B. A faulty 1157 bulb would only affect the front parking light and turn signal. The taillight would be unaffected.
- C. While a defective headlight switch or parking light relay could cause this, a specific break in the wiring to the left side is more localized and a common failure point than the entire switch failing for only one side's parking lights.
Shared Power Circuit Fault
A shared power circuit fault occurs when a single power wire or component supplies power to multiple lights or functions. A break in this shared path will cause all connected components to fail simultaneously.
- Parking lights and taillights often share a common power feed.
- Diagnosis involves tracing power from the switch/relay to the affected lights.
- Distinguished from ground faults by specific functions affected vs. general failure.
Memory trick: Parking's out? Power's in doubt!