Code C0661 means the ABS or chassis control module is detecting a lower-than-expected voltage signal from the left rear wheel speed sensor's secondary ('B') circuit. This almost always points to a problem with the sensor, its wiring, or the tone ring rather than the control module itself. When active, the fault normally disables or limits ABS, traction control, and stability control operation.
The sensor can fail internally from heat, moisture intrusion, or wear, producing a weak or absent signal that the module reads as a low-circuit condition.
Corroded, bent, chafed, or moisture-wicking connector pins, along with damaged harness wiring, can add resistance or short the signal to ground, causing a low-voltage reading.
A worn wheel bearing, bent sensor bracket, missing retaining clip, cracked tone ring, or incorrect sensor mounting can reduce the generated signal amplitude enough to set the code.
Metal debris, brake dust, corrosion, or road grime on the sensor tip or reluctor ring can weaken the signal and trigger a low-circuit fault.
If the sensor, wiring, and tone ring all test good, an internal issue in the ABS or chassis control module is possible, but it is the least likely cause for this code.
You can usually drive cautiously, but expect ABS, traction control, and stability control to be disabled or degraded. Avoid hard braking and slippery conditions until the fault is repaired.
Usually no. Most C0661 faults are caused by a failed sensor, damaged wiring, or a connector/tone-ring issue. Replacing the control module without first testing the sensor circuit is normally unnecessary and often wrong.
The 'B' designation typically refers to a secondary or redundant wheel-speed sensor circuit for that wheel, often found in integrated hub/bearing assemblies with dual sensors. It is not a second physical wheel.
You can clear it to verify it resets, but if the underlying sensor, wiring, or tone-ring fault remains, the code and warning lights will return, usually immediately or after driving a short distance.