Drivers first notice an illuminated ABS warning light on the instrument cluster. In many cases the light flashes intermittently, especially when the vehicle is turning or braking hard. Because the anti‑lock braking system (ABS) and electronic stability control (ESC) rely on each wheel’s speed data, the system may disengage traction‑control for the right rear wheel, resulting in a “reduced brake modulation” feel. The vehicle will still brake, but the ABS pump will not modulate pressure for that wheel, and the ESC may not intervene during loss‑of‑traction events.
The wheel‑speed sensor provides two signal circuits: primary (A) and secondary (B). The B circuit supplies a reference voltage used by the ABS module for fault detection. A broken wire, cracked insulation, or a poor ground can drop the voltage below the module’s threshold, triggering C0668.
The sensor’s connector is exposed to road spray, brake dust, and temperature cycles. Corrosion or a loose pin can create intermittent contact, producing a low‑voltage reading that the ABS module interprets as a circuit‑low condition.
Inside the sensor, the B‑signal coil can short or open due to moisture intrusion or mechanical damage. While the primary A‑signal may still transmit speed pulses, the secondary reference fails, resulting in the code.
Even with intact wiring and a healthy sensor, the ABS control module may lose the ability to read the B‑signal due to internal board damage, firmware corruption, or a failed communication line. The module then reports a circuit‑low condition.
– Use a factory‑level ABS/ESC scanner. Record any related codes (e.g., C0667, C0670) that can point to a broader module issue.
– Locate the right‑rear wheel‑speed sensor. Check the harness for frayed wires, chafing, or pinch points near the brake caliper and suspension components. Inspect the connector for corrosion, bent pins, or water intrusion.
– With the ignition on, measure the B‑signal voltage at the sensor connector (typically 5 V ± 0.5 V). A reading below 4 V indicates a low‑circuit condition.
– Perform a continuity check from the sensor B‑wire to the ABS module connector. Resistance should be below 1 Ω; higher values suggest wiring damage.
– Using a scope or a dedicated sensor tester, confirm that the primary A‑signal produces a clean square wave while the B‑signal remains steady. If the B‑signal is absent or noisy, the sensor’s secondary coil is suspect.
– Run the scanner’s “module communication” routine. Failure to communicate with the ABS control module, despite good wiring, points to a module fault.
– Wiring/Connector Issue: Repair or replace damaged harness sections, clean or reseat the connector, and re‑torque to manufacturer spec. Re‑scan; the code should clear.
– Sensor Secondary Failure: While the sensor itself is a component, the primary remedy is often to replace the sensor assembly. However, if the sensor is integrated with a control module (e.g., a combined wheel‑speed/steering‑angle unit), the module may need re‑programming.
– ABS Module Fault: If communication tests fail and wiring is sound, the ABS control module likely requires re‑programming or replacement. Re‑programming can resolve firmware corruption; persistent failures typically necessitate a replacement unit.
– Replacement ABS modules range from $600‑$900 for the hardware plus $200‑$300 labor.
– Re‑programming costs about $100‑$150 for the flash and verification.
– Ensure the new or re‑programmed module is VIN‑matched to maintain security and immobilizer compatibility.
– After repair, clear all codes and perform a road test. Confirm that the ABS/ESC warning lamp remains off during hard braking and cornering maneuvers.
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.