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Almost all modern vehicles come with vehicle dynamics systems, which, at the very least, include anti-lock brakes (ABS). For the ABS system to work, the vehicle dynamics module(s) need(s) wheel speed information for each of the four wheels.

The ABS system then uses wheel speed information to pulse the brakes during a sudden hard braking event to prevent wheel lockup, because when the wheels are sliding rather than rolling, you can’t steer the vehicle, and that’s dangerous. Most newer vehicles have vehicle stability systems with steering wheel rotation and vehicle yaw sensors so that during evasive maneuvers, the module will apply the brakes on just one wheel (depending on sensor inputs to keep the vehicle under the control of the driver. These are important systems designed to prevent accidents.

The ABS code C0040 is a diagnostic trouble code (DTC) that warns of a potential malfunction in one of the wheel speed sensors or the sensor’s circuit.

What Does the C0040 Code Mean?

The C044 code stands for “Right Front Wheel Speed Circuit Malfunction.” It is set when the ABS module notices that there is an issue with the right front wheel speed sensor or its circuitry.

The wheel speed sensors provide wheel speed input to the electronic module(s) that prevent wheel lockup and provide vehicle stability functions. There are usually four sensors, although some earlier ABS systems on rear wheel drive pickups and vans may simply measure rear axle speed or transmission output shaft speed. Cars always have four sensors if equipped with ABS, and vehicle dynamics systems always have a sensor at each wheel.

If you’re planning a DIY fix, you can read our technical discussion about wheel speed sensors.

Continue reading to learn the possible causes of C0040.

Note: The definition of code C0040 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What Are the Possible Causes of the C0040 Code?

As with most OBD-II codes, there are multiple possible reasons why the code C0040 may register in your vehicle’s memory. Below are just some of these causes:

abs warning light on
An illuminated ABS light is a common symptom of the C0040 code.

What Are the Common Symptoms of the C0040 Code?

If you’re dealing with the C0040 code, you might deal with the following signs and symptoms as well.

  • Illuminated ABS light
  • Other illuminated warning lights (i.e. traction control)
  • Inoperative ABS system

How to Diagnose the C0040 Code

Troubleshooting OBD-II codes such as the code C0040 may sound intimidating, but it helps if you have the proper tools and information sources.

As a seasoned DIYer, you may refer to a handy repair manual or database for information. Consulting professional mechanics and technicians help, too. There are other sources and guides out there you can use for more information. Below is a helpful video resource on the C0040 code that you can use:

How to Fix the C0040 Code

There is no one-size-fits-all solution for OBD-II codes such as the C0040 because vehicles have different configurations and repair instructions, depending on their make and model. However, C0040 repair doesn’t have to become complicated, as there are a lot of resources available such as repair manuals and databases. If you’re not that confident in your own repair skills, you might want to leave the repairs to a professional so you can avoid more issues down the line.

Other Notes About C0040

C0040 has high repair importance and difficulty level. This means that an amateur DIYer may lack the ability to address the issue properly without the proper tools and information. Problems with the wheel speed sensor may not affect your brake function, but it can affect handling in wet, slick, or icy road conditions. This is because the wheel speed sensors are also a primary input to the traction control and stability control systems.

To prevent any further damage to the sensor or its related components, be vigilant and quick to address the issue immediately.

A Closer Look at Wheel Speed Sensors

Older wheel speed sensors are usually located outside the wheel hub. Like any other speed sensor, this can be an inductive sensor (magnet with a wire coil surrounding it) that produces an AC analog wave, or it can be digital, so that it produces a square wave signal. Some square wave wheel speed sensors can even detect whether the vehicle is moving forward or backward.

The sensor typically either reads steel teeth moving past the sensor as the wheel spins (this is very common) or in some cases, a series of small, carefully spaced magnets will be embedded in the bearing grease seal and the sensor will be designed to read those magnetic pulses. There are times when the magnets embedded within the bearing grease seal may cause a wheel speed sensor code, and in that case, the bearing must be replaced.

wheel hub with abs sensor
Older wheel speed sensors are usually located outside the wheel hub. You can often find modern wheel speed sensors in the wheel hub assembly.

The information that the sensors generate is used by the ABS computer or module to control the ABS system. It may also be used by other modules, however, this depends on the make and model of the vehicle.

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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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