Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairEasy
A technician is diagnosing an intermittent ABS warning light on a vehicle. The scan tool shows a C0035 fault code for the right front wheel speed sensor. Live data shows the right front wheel speed sensor reading 0 MPH while the vehicle is moving at 15 MPH. All other wheel speed sensors are reading correctly. Which of the following is the MOST likely cause of this issue?
- AA defective ABS control module
- BA faulty ABS hydraulic control unit
- CAn open circuit in the right front wheel speed sensor wiring
- DLow brake fluid level in the master cylinder
Show answer & explanationAnswer & explanation
Correct answer: C. An open circuit in the right front wheel speed sensor wiring
A wheel speed sensor reading 0 MPH when the vehicle is moving indicates a loss of signal from that specific sensor. An open circuit in the wiring would prevent the signal from reaching the ABS control module.
Why the other options are wrong
- A. A defective ABS control module would typically result in more widespread ABS malfunctions or different diagnostic trouble codes affecting multiple sensors or system functions.
- B. A faulty ABS hydraulic control unit would likely cause issues with brake pressure modulation or multiple wheel circuits, not just a single sensor's signal.
- D. Low brake fluid primarily affects the hydraulic braking system's ability to apply pressure and would typically trigger a brake warning light, not a specific wheel speed sensor fault.
Wheel Speed Sensor Open Circuit
An interruption in the electrical path of a wheel speed sensor, preventing it from sending a signal to the ABS control module.
- Causes a 0 MPH reading from the affected sensor when the wheel is rotating.
- Often triggers specific diagnostic trouble codes (DTCs) for that sensor.
- Can be caused by damaged wiring, corroded connectors, or a faulty sensor itself.
Memory trick: When the wheel stops sending speed, check the wiring for a break in the lead!