Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairEasy
A technician is diagnosing a vehicle with an ABS warning light illuminated. The scan tool reports a C0040 fault code, indicating a 'Right Front Wheel Speed Sensor Circuit Malfunction'. Upon inspection, the technician finds the wheel speed sensor wiring harness appears visually intact, but an ohmmeter test across the sensor terminals shows an open circuit. What is the MOST likely cause of this fault?
- ALow system voltage to the ABS control module
- BInternal open circuit within the wheel speed sensor
- CExcessive air gap between the sensor and tone ring
- DCorrosion in the ABS control module connector
Show answer & explanationAnswer & explanation
Correct answer: B. Internal open circuit within the wheel speed sensor
An ohmmeter test showing an open circuit directly indicates a break in the internal wiring or components of the sensor itself, which is a common failure mode for wheel speed sensors.
Why the other options are wrong
- A. Low system voltage affects the module's power supply, not the internal circuit integrity of an individual wheel speed sensor.
- C. Excessive air gap typically leads to a weak or no signal, not an open circuit as measured by an ohmmeter.
- D. Corrosion in a connector would likely show high resistance or an intermittent connection, not a complete open circuit within the sensor.
Wheel Speed Sensor Open Circuit
An open circuit in a wheel speed sensor means there is a break in its internal wiring or coil, preventing electrical current flow and signal generation.
- Causes a 'no signal' condition for the ABS module.
- Diagnosed with an ohmmeter showing infinite resistance.
- Often leads to an ABS warning light and specific fault codes.
Memory trick: Don't Open the Wheel, it's a Sensor's Secret!