C0040 Code: Right Front Wheel Speed Circuit Malfunction

What the C0040 code means

The ABS module compares the speed of all four wheels to detect when one is about to lock up during braking and then modulates the brake pressure at that wheel.

The C0040 code means the signal from the right front wheel is missing, intermittent or does not agree with the other wheels. Without that data the system cannot step in safely, so it disables itself and turns on the warning light.

Because of where it sits in the suspension, this sensor and its wiring work in constant motion and are exposed to water, mud and road salt. That is why a cable that is broken inside its insulation is as common a cause as the sensor itself.

It helps to know the difference between the two types of sensor this circuit can use, because it completely changes how you test it. A passive sensor generates its own signal by induction and is measured with an ohmmeter, and it needs no supply. An active sensor needs voltage from the ABS unit and returns a switched current signal that cannot be judged with a simple resistance measurement.

Active sensors, which are the usual kind on modern vehicles, have an important advantage: they work from very low speeds, almost from the moment the vehicle starts to move. That is why on these systems the ABS can step in during slow maneuvers, and why a failure is noticed sooner.

C0040 code symptoms

• ABS warning light on, along with the stability control and traction control lights.
• The ABS does not act when you brake on a slippery surface.
• An erratic speedometer on some vehicles.
• Cruise control disabled.
• A brief ABS activation at low speed for no reason.
• The light goes out after a restart and comes back after a few yards of driving.
• Noise from the hydraulic unit at startup when you are not braking.

Likely causes of the C0040 code

1. Wiring broken inside its insulation in the area where the suspension moves.
2. Failed wheel speed sensor from internal moisture.
3. Metal shavings stuck to the sensor tip.
4. Tone ring that is rusty, dirty or has damaged teeth.
5. Connector with water or corrosion.
6. Wheel bearing with play, which changes the distance to the sensor.
7. Defective ABS module. An uncommon cause that is ruled out last.
8. Magnetic ring in the bearing that has lost its magnetism because magnetic tools were used near it.
9. Bearing installed incorrectly, with the magnetized face toward the wrong side.
10. Rust built up between the sensor and its bore, which increases the air gap.

How to diagnose the C0040 code step by step

1. Compare all four wheel speeds in live data while driving slowly. The one that stays at zero or jumps around is the affected one.
2. Inspect the cable along its whole length, flexing it by hand to find breaks inside the insulation.
3. Clean the sensor tip and check for metal shavings stuck to it.
4. Inspect the tone ring for rust or broken teeth. On many vehicles it is built into the bearing.
5. Measure the resistance on passive sensors and compare it with the specification.
6. Check the signal with an oscilloscope while turning the wheel by hand.
7. Check the bearing play with the vehicle lifted.
8. Find out whether the sensor is passive or active before measuring. A passive one is checked with an ohmmeter, while an active one needs a supply and a current measurement.
9. Check the real air gap, keeping in mind that rust built up in the bore increases it without being obvious to the eye.
10. If the bearing was replaced recently, verify its orientation. Installed backwards, the magnetized face ends up on the wrong side and there is no signal.

How to fix the C0040 code

• Clean the sensor and its bore if there is dirt or metal shavings.
• Replace the sensor when its measurements are out of range.
• Repair the wiring along its original path so it does not rub again, using flexible protection.
• Clean or replace the corroded connector.
• Replace the wheel bearing when the magnetic ring is built into it and is damaged.
• Clean the sensor bore until the rust is gone, since it separates the sensor from the ring and weakens the signal.
• Check the bearing orientation if it was replaced recently. Installed backwards, it produces no signal even when it is new.
• Keep magnetic tools away from the ring during the repair, because they can demagnetize it permanently.

After the repair, clear the codes and drive a short stretch. The ABS light only goes out when the system verifies all four signals in motion.

Technical reference data

• Passive inductive sensors: typical resistance between 800 and 2,000 ohms, with no external supply.
• Active Hall effect sensors: 8 to 12 volt supply and a switched current signal between 7 and 14 milliamps.
• Usual air gap between sensor and ring: 0.4 to 1.5 millimeters (about 0.016 to 0.059 inches).
• The ABS starts comparing signals above roughly 5 to 10 km/h (about 3 to 6 mph).
• Active sensors work from very low speeds. Passive ones need at least about 5 km/h (about 3 mph).

Brand-specific notes

When the C0040 code comes and goes depending on the road or when turning, suspect the wiring before the sensor. A conductor broken inside the insulation only shows up when the suspension moves.

The magnetic ring built into the bearing loses its magnetism if magnetic tools are brought near it during installation. It is an easy mistake to make, and it leaves a brand new bearing useless for the ABS even though it spins perfectly.

Frequently asked questions about C0040