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C0040 Severity
Once the vehicle's onboard system no longer detects the issue, most codes will disappear on their own. If your vehicle still has this code, however, it should be properly diagnosed to avoid a more serious problem.
For a proper diagnosis, RepairPal Certified shops have the equipment, tools, and expertise.
Moderate severity codes might not threaten the vehicle's safety or functionality immediately, but you should still get it diagnosed within a reasonable time frame to keep your car running well.
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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
Code C0040 indicates the ABS module has determined a problem with the right front wheel speed sensor or its circuit.
Get it diagnosed by a professional
Code C0040 is typically caused by one of the following:
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.
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.
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.
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.
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.
C0040 is often set with sub-codes which can help with diagnosis.