C0040 is a chassis trouble code that occurs in the Buick LeSabre. GM designated C0040 to indicate an issue with the right front wheel speed sensor.
Replacing your LeSabre’s right front wheel speed sensor is the most common repair for C0040, but it’s not the only one. Wiring harness issues are common with this code.
C0040 Definition
C0040: Right Front Wheel Speed Sensor
C0040 indicates that your LeSabre’s right front wheel speed sensor is malfunctioning. Here’s the definition, broken down into three parts:
Right Front
Even if you are a pretty experienced shade tree mechanic, you may wonder which side of the vehicle Buick considers the “right front.”
Auto manufacturers use left and right instead of “passenger” and “driver” since the side of the vehicle people drive on can change depending on what part of the world they live in.
LF is determined by where it is in relation to you when sitting in your LeSabre’s driver’s seat. So, the “right front” is the wheel that’s ahead of you and to the right.
Wheel Speed Sensor
Wheel speed sensors measure how fast each wheel is turning. The Buick LeSabre uses that data to determine if one of its wheels has broken traction or if the brakes have locked up.
The wheel speed sensors help the ABS module determine if one of the wheels has broken traction. If your vehicle has traction control, that system uses the wheel speed sensors as well.
Qualifiers
Qualifiers at the end of the code can help you further determine what is going on with the wheel speed sensor.
For example:
- C0040-00: Right Front Wheel Speed Sensor Circuit
- C0040-0F: Right Front Wheel Speed Sensor Signal Eratic
If you get the 0F version of this code, you’d definitely want to start by taking a look at the wiring harness coming from your LeSabre’s right front wheel speed sensor.
Or, if you have a good scan tool, you can look at the signal waves coming from the LF wheel speed sensor and compare the values to the RF wheel speed sensor.
When the left front wheel speed sensor goes bad, the code is C0035. If they’re both the same part number (and they should be). You can swap them with each other and see if the code “jumps” to the other wheel.
Buick LeSabre C0040 Symptoms
Since this is a chassis code, there’s no risk of your LeSabre’s engine breaking down with it. But, ABS and traction control are important safety features, and this code should not be ignored. Here are the most common C0040 symptoms:
- Check engine light
- ABS system inactive (ABS light)
- Traction control disabled
Buick LeSabre C0040: Causes and Diagnosis
Here are the most common causes of C0040 in the Buick LeSabre, and a good order to diagnose them in:
Wheel Speed Sensor Wiring
Check the wiring harness going to the right front wheel speed sensor. Is it damaged in any way? This is a very common cause of C0040. You’ll be looking for cracked, burnt, or frayed wires.
The wiring can go bad even without breaking. It’ll be “floppier” or more pullable (very gently) than the rest of the wiring in the harness. You can see exactly what that looks like in the video below.
Wheel Speed Sensor
Replacing the wheel speed sensor is the most common repair when you get C0040 in the Buick LeSabre.
Wheel speed sensors can and do go bad. They can also get too grimy to do their job. Sometimes cleaning them will clear the code.
Technical Service Bulletins
TSBs are repair information that comes directly from Buick. You can get these directly from the NTSB website: https://www.nhtsa.gov/recalls
You can search by VIN or by model year.
These bulletins detail problems known to GM about the LeSabre, and the common repair steps. Even if there is a TSB related to your vehicle, it will still often end up being the wheel speed sensor that is causing C0040.
Other Causes
- Reluctor wheel
- Abs Module
Conclusion
The most common cause of C0040 is a bad wheel speed sensor. Good luck repairing your LeSabre!