C0039 indicates that the ABS or stability control system has detected a fault with the right rear tone wheel, also known as the reluctor ring. This toothed or magnetic ring works with the wheel speed sensor to generate a signal indicating wheel rotation. A cracked, dirty, or misaligned tone wheel can cause erratic or missing signals, triggering this code.
The reluctor ring may have cracked, chipped, or heavily corroded teeth that prevent the wheel speed sensor from generating a consistent signal.
Metallic debris, mud, or rust buildup can disturb the magnetic field and produce an erratic pattern that the system interprets as a tone wheel fault.
Excessive bearing play can allow the tone wheel to wobble or shift, altering the air gap between the tone wheel and sensor beyond acceptable limits.
A failed sensor, loose mounting bolt, or improper clearance can cause signal dropouts that the system attributes to the tone wheel rather than the sensor itself.
Corroded, chafed, or poorly connected wiring in the right rear speed sensor circuit can introduce electrical noise resembling a tone wheel failure.
In rare cases the ABS controller may incorrectly interpret a good signal; this should only be considered after the tone wheel, sensor, bearing, and wiring have been thoroughly verified.
Standard braking usually remains functional, but ABS and stability control may be disabled. This can increase stopping distances on slippery surfaces, so the fault should be diagnosed and repaired promptly.
Inspect the tone wheel first for physical damage or debris. If the tone wheel is intact and properly aligned, test the sensor and its air gap before replacing either component.
Almost never. C0039 is almost always caused by a mechanical tone wheel issue, a sensor problem, or wiring damage. Replacing the PCM or ABS module is rarely the correct repair.
Not necessarily, but a worn bearing can cause tone wheel wobble that sets this code. Always check the bearing for play and roughness during diagnosis.