C0039: Cible de capteur de roue arrière droite

The right rear tone wheel is faulty.

A wheel speed sensor and the ring it reads are two parts, and the codes distinguish them.

The sensor is a fixed pickup; the tone wheel is a toothed or magnetically striped ring that turns with the hub. The sensor counts teeth going past. So an identical symptom — no signal, or a signal that goes wrong at certain speeds — can come from either half, and replacing the sensor when the ring is at fault is one of the commonest wasted repairs on this system.

Look at the ring before buying the sensor. Rust swelling under a pressed-on ring, a cracked ring, a tooth knocked off during a hub or driveshaft job, or dirt packed into the teeth all produce sensor-shaped faults. Iron dust clinging to a magnetic sensor tip does the same and wipes off with a rag.

A worn wheel bearing belongs on this list too, because many sensors are built into the bearing and because play at the hub changes the gap between sensor and ring as the wheel turns.

This code names the ring rather than the sensor, which is the useful part. The module has concluded the signal pattern is wrong in a way a healthy ring would not produce — teeth missing, spacing irregular, or the ring turning eccentrically.

Inspect before ordering anything. A ring pressed onto a hub can be lifted and cracked by rust beneath it; a tooth can be knocked off refitting a driveshaft; and a magnetic encoder ring — a black plastic-looking band on the back of a bearing seal — is easily wiped, scratched or demagnetised during a bearing change.

How ABS actually works, because the codes in this range name its parts.

A wheel speed sensor at each corner tells the module how fast that wheel is turning. When one decelerates far faster than the others under braking, it is about to lock. The module then does three things in sequence, many times a second: it isolates that wheel by closing an inlet valve so no more pressure can reach it, it dumps pressure by opening an outlet valve, and then it rebuilds pressure using the pump. That cycle is the shudder you feel through the pedal.

Every element of that loop has its own code here, which is what makes this range diagnosable rather than a single "ABS fault" — the valves at C0010 onward, the pump at C0020, the sensors at C0030 onward.

The most important thing first: the car still stops.

ABS, traction control and stability control are all additions to a braking system that works without them. When the module finds a fault it switches itself off and hands back a conventional, fully hydraulic brake system — the pedal, the master cylinder, the lines and the calipers are untouched. You have not lost your brakes.

What you have lost is the help. Without ABS the wheels can lock under hard braking, which lengthens the stopping distance on a slippery surface and — the part people forget — takes away your ability to steer while braking hard, because a locked wheel cannot generate cornering force. Without stability control the car will not catch a slide for you.

So drive it, but drive it as though it were forty years old: brake earlier, leave more room, and be especially careful in rain, snow and on gravel.