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What does code C0040 mean?

Code C0040 stands for Right Front Wheel Speed Sensor Circuit

Wheel speed sensors (WSS) do exactly what their name implies – measure wheel speed. This information is used by the antilock brake system (ABS) computer to determine control of the ABS system. Wheel speed information may also be used by other onboard computers (referred to as modules), depending on the vehicle. There are typically four wheel speed sensors, one at each wheel.

Most WSS sensors produce a voltage in relation to the changing magnetic field that occurs as the wheel rotates. A toothed ring (called a reluctor) is mounted on the axle shaft or wheel hub, which rotates with the wheel assembly. As the rings turns past the WSS, it causes the sensor to produce an alternating current (AC) voltage. The frequency and amplitude of the sensor signal increases as the wheel turns faster/

wheel speed sensor

Wheel speed sensor

Code C0040 indicates the ABS module has determined a problem with the right front wheel speed sensor or its circuit.

C0040 Symptoms

  • An illuminated ABS light
  • Other illuminated warning lights such as traction control
  • An inoperative ABS system
Get it diagnosed by a professional

Common causes for C0040

Code C0040 is typically caused by one of the following:

  • A faulty wheel speed sensor
  • A problem with the wheel speed sensor circuit
  • Reluctor issues
  • A problem with the ABS module

How to diagnose and repair C0040

Perform a preliminary inspection

Sometimes C0040 can pop up intermittently. Clear the code and see if it returns. If it does, the next step is to perform a visual inspection. A trained eye can check for issues such as broken wires and loose connections. The condition of the reluctor and its mounting should also be checked. Issues such as chipped reluctor teeth or a worn wheel bearing can cause code C0040. If a problem is found, the issue should be repaired and the code cleared. If nothing is discovered, check for technical service bulletins (TSBs). TSBs are recommended diagnostic and repair procedures put out by the vehicle manufacturer. Finding a related TSB can greatly reduce diagnostic time.

Scan tool check

Typically, a technician will begin by checking WSS operation with a diagnostic scan tool. This tool connects directly to the vehicle’s diagnostic port, where it can gain access to vehicle data. Then the vehicle can be driven while monitoring wheel speed sensor values. The value for the right front wheel speed sensor should increase with vehicle speed, just like the other wheel speed sensors. If the value does not increase, there is a problem with the WSS or its circuit.

Check the WSS

The next step is to determine if the problem is with the WSS itself or the circuit. A technician will test the WSS directly using a digital multimeter (DMM) or digital storage oscilloscope (DSO). Either can be connected directly to the WSS while turning the wheel by hand. As the wheel is turned, the WSS should produce an AC voltage reading (typically between 50 to 700 MV). If it does not, the sensor is faulty and should be replaced.

Check the WSS circuit

At this point, the WSS sensor performed OK when tested directly, but did not register properly on the scan tool. This indicates the WSS signal is not registering at the ABS module, potentially due to a problem with the circuit. The circuit can be tested for excessive resistance, shorts and opens using a DMM. If a problem is found, the factory wiring diagram will need to be traced to pinpoint the concern. Then any necessary repairs can be made.

Check the ABS module

In very rare instances, the ABS module may not be reading the WSS properly. In this case, the module would need to be either reprogrammed or replaced.

Other diagnostic codes related to C0040

  • C0035: Code C0035 indicates the ABS module has detected a problem with the left front wheel speed sensor.
  • C0045: Code C0045 indicates the ABS module has detected a problem with the left rear wheel speed sensor.
  • C0050: Code C0050 indicates the ABS module has detected a problem with the right rear wheel speed sensor.
  • C0055: Code C0055 indicates the ABS module has detected a problem with the rear wheel speed sensor circuit.

Code C0040 technical details

C0040 is often set with sub-codes which can help with diagnosis.

  • C0040 02: Code C0040 02 indicates the ABS module has detected a short to ground in the right front wheel speed sensor circuit.
  • C0040 05: Code C0040 05 indicates the ABS module has detected a short to power or an open in the right front wheel speed sensor circuit.
  • C0040 0F: Code C0040 0F indicates the ABS module has detected erratic operation from the right front wheel speed sensor.
  • C0040 18: Code C0040 18 indicates the ABS module has detected the right front wheel speed sensor signal amplitude is below minimum.
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