ASE G1 Auto Maintenance and Light RepairBrakesHard
A technician is diagnosing an Antilock Brake System (ABS) with an illuminated warning light. The scan tool shows a fault code indicating a 'Left Front Wheel Speed Sensor Open Circuit'. Which of the following is the MOST likely cause?
- AA broken or disconnected wheel speed sensor wire
- BA low brake fluid level in the master cylinder
- CA faulty ABS hydraulic control unit (HCU)
- DA seized brake caliper on the left front wheel
Show answer & explanationAnswer & explanation
Correct answer: A. A broken or disconnected wheel speed sensor wire
An 'open circuit' fault code specifically indicates a break or disconnection in the electrical wiring for the sensor. While the sensor itself could be faulty, a broken wire or connector issue is a very common cause of an open circuit, preventing the sensor from sending its signal to the ABS control module.
Why the other options are wrong
- B. Low brake fluid typically triggers a brake warning light, not an ABS light, and would not cause a wheel speed sensor open circuit fault.
- C. A faulty HCU would likely result in multiple or different ABS codes, not specifically an open circuit for a single wheel speed sensor.
- D. A seized brake caliper would cause braking issues (like pulling or dragging) but would not directly lead to an 'open circuit' fault code for a wheel speed sensor.
Wheel Speed Sensor Open Circuit
A diagnostic trouble code (DTC) indicating a break in the electrical connection or wiring to a specific wheel speed sensor.
- Prevents the ABS module from receiving speed data for that wheel.
- Common causes include damaged wiring, corroded connectors, or a faulty sensor.
- Requires electrical diagnosis to pinpoint the exact location of the open circuit.
Memory trick: Open circuit means the electrical path is broken, like a snapped wire.