Tucked at each wheel is a small sensor that quietly makes anti-lock brakes, traction control, and stability control possible. The wheel speed sensor is one of the most common brake-electronics parts to fail, and when it does the symptoms are unmistakable. Here is what it does.
What it does
A wheel speed sensor measures how fast each individual wheel is rotating and reports that to the ABS module many times a second. The module compares all four wheels; if one suddenly slows much faster than the others during braking, it knows that wheel is about to lock and pulses the brake to prevent it - see how ABS works. The same signal feeds traction and stability control.
How it works
Most sensors work magnetically. A toothed ring (tone ring or reluctor) spins with the wheel or axle, and the sensor generates a pulsing signal as each tooth passes - the faster the wheel, the faster the pulses. Modern active sensors use a powered hall-effect element for accuracy even at very low speed. Either way it is a simple, rugged part - but it lives in a filthy, wet environment.
Why it fails and the symptoms
Sensors fail from corrosion, a damaged tip, metal debris stuck to the magnet, or a broken wire from constant flexing. A damaged tone ring causes the same fault. The classic symptom is an ABS warning light, often with the traction control light too, and loss of ABS/traction function - your normal brakes still work, but the safety assists go offline.
Diagnosis and fix
A scan tool reads a code pointing to the exact corner, which makes diagnosis straightforward. Often the fix is as cheap as cleaning debris off the sensor tip; sometimes the sensor or tone ring is replaced. It has nothing to do with pad wear, but while the wheel is off it is a good time to check the pads and refresh hardware.