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DTC codes / C06xx · Brakes, steering and suspension / C0661

C0661Left Rear Wheel Speed Sensor B Circuit Low

The C0661 code indicates that the left rear wheel speed sensor circuit is reporting a low voltage condition, which the antilock braking system (ABS) control module interprets as a fault. The computer has detected that the signal voltage from that sensor is below the expected threshold, potentially due to a short to ground, a faulty sensor, or a wiring issue.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaBrakes, steering and suspension

Symptoms you may notice

  • ABS warning light illuminated on the dashboard
  • Traction control system disabled or warning light on
  • Often no symptoms besides the warning light
  • Intermittent or erratic ABS operation at low speeds
  • Possible reduced braking performance in slippery conditions due to ABS deactivation

Common causes, most likely first

  1. Faulty left rear wheel speed sensor
  2. Damaged or corroded wiring harness or connector at the sensor
  3. Short to ground in the sensor circuit
  4. Debris or metallic particles on the sensor tip or tone ring
  5. Worn or damaged tone ring (reluctor ring) on the axle
  6. Moisture intrusion into the sensor connector
  7. ABS control module internal fault (less common)

How to diagnose and fix it

  1. Inspect the left rear wheel speed sensor and its wiring for visible damage, corrosion, or debris
  2. Clean the sensor tip and tone ring of any metallic debris or dirt
  3. Disconnect the sensor connector and check for moisture or corrosion; clean and apply dielectric grease
  4. Measure the sensor resistance and compare to manufacturer specifications
  5. Check the sensor circuit for a short to ground using a multimeter and wiring diagram
  6. Replace the left rear wheel speed sensor if faulty
  7. Clear the code and test-drive the vehicle to verify the repair

Common misdiagnosis of C0661

  • A low voltage code often results from a short to ground in the wiring, not a failed sensor, so always inspect the harness before replacing parts.
  • Metallic debris on the sensor tip can mimic a low voltage signal by disrupting the magnetic field, so cleaning may resolve the issue without replacement.
  • The tone ring may be cracked or missing teeth, which can cause erratic signals that set this code, so inspect it carefully.
  • Corroded connectors or chafed wires near the wheel arch are frequent culprits, especially in areas with road salt exposure.
  • A weak battery or charging system issue can cause multiple wheel speed sensor codes; check system voltage first.

Can I keep driving with C0661?

You can drive the vehicle with this code, but the ABS and traction control systems may be disabled, reducing safety in emergency braking or slippery conditions. Avoid hard braking and slippery roads until repaired.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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