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C0040 – What Does It Mean and How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


C0040 is an OBD-II Code that refers to Right Front Wheel Speed Circuit Malfunction


A C0040 code is a ABS diagnostic trouble code that occurs when the module detects a signal voltage input indicating the Right Front Wheel Speed Sensor has experienced a malfunction in its circuitry which does not fall in line with the manufacturer’s specified parameters. 

The Electronic Brake Control Module (EBCM) activates the wheel speed sensors with an ignition signal and deliver a square wave signal to the control module. The signal from the ECM is understood by the EBCM to determine the speed of the wheel’s rotation.

When an issue occurs with the circuit or sensor, it will set off the check engine light and set the code C0040.


Common causes for this code include:

  • Wheel speed sensor is faulty
  • Wheel speed sensor circuit problems
  • Reluctor or wheel bearing is bad
  • Issues with the ABS module


The symptoms of a C0040 code are:

  • Illuminated Check Engine Light 
  • Illuminated ABS light
  • Incorrect vehicle speed displayed on the dashboard.
  • The brake switch receives a few jolts.


To diagnose a C0040 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Visually inspect the wiring and connectors for damage.
  3. Check the resistance of the wheel speed sensor.
  4. Test the signal at the ABS module.
  5. Check to see if the reluctor or wheel bearing is bad.
  6. Check for voltage at the brake switch.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a C0040 code:

  • Not thoroughly inspecting all the wiring and connectors.
  • Not checking the resistance of the wheel speed sensor.
  • Not testing the signal at the ABS module.

How serious is this?

A C0040 code is a serious code and should be fixed as soon as possible.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Repair or replace any faulty parts such as wiring, connectors, or hoses.
  • Replace the wheel speed sensor.
  • Repair or replace the ABS module.

Related codes

A C0040 is related to and may be accompanied by the following codes:

C0035 – Left Front Wheel Speed Sensor Circuit Malfunction

C0041 – Right Rear Wheel Speed Sensor Circuit Malfunction

C0045 – Left Rear Wheel Speed Sensor Circuit Malfunction 

B2799 – Lost Communication with ECM/PCM “A” Bus


In conclusion, the C0040 code is a diagnostic trouble code that refers to a problem with the Right Front Wheel Speed Sensor Circuit. Anytime this code is present, it is important to have the vehicle diagnosed and repaired as soon as possible to avoid any further damage to the vehicle.