C003A: Right Rear Wheel Speed Sensor
What to do
Your ordinary brakes still work — this is the anti-lock assistance switching off, not the brakes failing. Stopping distances get longer on slippery surfaces and the wheels can lock under hard braking, which also takes away your steering while you brake. Leave more room, brake earlier, and take extra care in rain, snow or on gravel. Have it looked at soon; in most countries the car will fail its roadworthiness test with this light on.
What this code means
The right rear wheel speed sensor circuit has a fault.
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.
Where these sensors live is why they fail. A wheel speed sensor sits at the hub, behind the wheel, in the direct path of everything the road throws up — water, salt, grit and kerb impacts — and its cable flexes with the suspension and steering on every journey. The cable and its connector fail far more often than the sensor itself.
Flex the harness by hand with the reading live if you can. An intermittent fault that appears over bumps or on full lock is a broken conductor inside intact-looking insulation, and no static resistance check will find it.
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.
Likely causes
Start here
- Faulty wheel speed sensor · specialist
- Corroded or loose connector · specialist
- Damaged wiring harness · specialist
- Metal debris on the sensor tip · specialist
Less common
- Damaged reluctor or tone ring · specialist
- Worn wheel bearing · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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