C0665 indicates the Anti-lock Brake System (ABS) module has detected that the left rear wheel speed sensor B does not match the expected signal characteristics for the vehicle. This typically means an incorrect or incompatible sensor was installed, or the sensor is malfunctioning in a way that produces abnormal readings.
A mechanic or previous repair installed a sensor with incorrect specifications for that wheel position (wrong part number, incompatible brand, or B-circuit sensor where A-circuit was expected).
The sensor itself has developed an internal fault causing signal output outside normal parameters, triggering the component mismatch detection.
Damaged wiring, corroded connectors, or poor electrical connections to the sensor can cause signal anomalies that the module interprets as incorrect component installation.
Incorrect sensor mounting gap or a damaged tone ring (absent/broken teeth) can produce abnormal wheel speed signals.
Cross-talk between sensor circuits or water/contaminants in the connector can create signal patterns that don't match expected component data.
Rarely, the ABS control module may have software that incorrectly identifies a otherwise correct sensor as incompatible; rarely the module itself has failed.
Almost never. The ABS module is very rarely the cause of C0665. In the vast majority of cases, replacing the sensor or repairing wiring resolves the issue. Only after verifying the correct sensor is installed, wiring is intact, and the sensor signals are within spec should module replacement be considered.
No. The sensor must match the exact specifications for your vehicle, including the correct circuit (A or B if equipped with dual sensors), connector type, and signal characteristics. Using an incorrect sensor will trigger this code.
The vehicle will typically still drive, but the ABS, traction control, and stability control systems will be disabled or limited. This reduces braking performance on slippery surfaces and increases stopping distances in emergency situations.
Replacing the engine computer (PCM/ECM) is almost never the correct fix for C0665, as this code relates specifically to the ABS wheel speed sensor circuit. Some shops may incorrectly assume the computer is at fault when they cannot find the actual sensor or wiring problem. Always verify sensor and wiring before considering module replacement.