U140B Code: Understanding an Implausible Left Rear Wheel Speed Signal

When your vehicle throws a U140b code, it signals a communication issue related to your car’s crucial safety systems. Specifically, the code “U140B Implausible Left Rear Wheel Speed Signal Received” indicates that your Transmission Control Module (TCM) is receiving an incorrect or unreliable signal from the Anti-lock Braking System (ABS) module regarding the left rear wheel speed. This doesn’t necessarily mean there’s a problem directly with the wheel speed sensor itself, but rather a breakdown in communication over the Controller Area Network (CAN) C circuit.

Decoding the U140B Trouble Code

The U140B code is closely related to other similar codes like U1417 (Implausible Left Wheel Distance Signal) and U140C (Implausible Right Rear Wheel Speed Signal). These codes point towards a broader issue within the vehicle’s network communication, specifically concerning wheel speed data transmitted from the ABS module.

It’s important to understand that while your standard OBD-II scanner might pick up the U140B code from the Powertrain Control Module (PCM) or TCM, the root cause often lies within the ABS module itself. The TCM relies on accurate wheel speed information from the ABS to function correctly, especially for systems like traction control, stability control, and even gear shifting in some vehicles. When the TCM detects an “implausible” signal – meaning a signal that is inconsistent, erratic, or simply doesn’t make sense – it sets the U140B code.

Potential Causes of a U140B Code

Several factors can trigger a U140B code. Here are some of the most common culprits:

• ABS Wheel Speed Sensor Fault: The left rear wheel speed sensor itself could be failing, damaged, or dirty. This sensor is responsible for monitoring the wheel’s rotational speed and sending that data to the ABS module.
• Wiring Harness or Connector Issues: Problems in the wiring harness or connectors related to the wheel speed sensor or the CAN Bus are frequent causes. This includes:
  • Short circuits: Wires may be damaged and shorting to ground or other wires.
  • Open circuits: Wires may be broken or disconnected, interrupting the signal flow.
  • Corrosion: Connectors can corrode, leading to poor electrical contact.
• CAN Bus Problems: The CAN Bus is the communication network that allows various modules in your car to “talk” to each other. Issues within the CAN C circuit can disrupt communication between the ABS module and the TCM.
• ABS Module Malfunction: In some cases, the ABS module itself might be faulty and sending incorrect data or failing to communicate properly.
• Recent Accidents or Damage: If your vehicle has been in a recent accident, particularly one affecting the rear axle or wheel area, the wheel speed sensor or related components could have been damaged.

Diagnosing and Resolving the U140B Code

Effectively diagnosing a U140B code requires a systematic approach and the right tools. Here’s a step-by-step guide:

1. Use a Diagnostic Scanner: The first and most crucial step is to use a professional-grade diagnostic scanner that can read codes from all modules in your vehicle, especially the ABS module. A basic OBD-II scanner might only show you the U140B code from the PCM or TCM, missing critical ABS codes.

2. Check for ABS Codes: Specifically, look for codes stored within the ABS module. These codes (often starting with “Cxxxx”) will provide more direct information about potential issues with the wheel speed sensors, ABS module, or related circuits. Codes related to the left rear wheel speed sensor or CAN communication are particularly relevant.

3. Inspect Wheel Speed Sensor Wiring and Connectors: Carefully examine the wiring harness and connectors leading to the left rear wheel speed sensor. Look for any signs of damage, such as:

   • Frays, cuts, or abrasions in the wiring.
   • Loose or corroded connectors.
   • Wires that are stretched or pulled tight.

4. Inspect the Wheel Speed Sensor: Visually inspect the sensor itself for any physical damage. Depending on your expertise and tools, you might also test the sensor’s resistance or output signal, but this often requires specialized equipment and knowledge.

5. Consider Recent Repairs or Accidents: If the U140B code appeared after recent repairs or an accident, re-examine the work done in those areas to see if anything could have disturbed the wheel speed sensor or its wiring.

6. Professional Diagnosis: If you are not comfortable performing these diagnostic steps yourself, or if you are unable to pinpoint the problem, it is highly recommended to take your vehicle to a qualified automotive technician. They have the advanced diagnostic tools and expertise to accurately diagnose and repair complex issues like CAN Bus communication faults and ABS module problems.

Addressing Limp Mode

The original article mentions “Limp Mode.” If the ABS module is reporting faults for multiple wheel speed sensors, or if the system detects a significant safety concern, your vehicle might enter limp mode. This is a safety feature that limits engine power and vehicle speed to prevent further damage or accidents. If your car is in limp mode, addressing the U140B code and any related ABS faults becomes even more critical.

In conclusion, the U140B code signifies a communication problem regarding the left rear wheel speed signal, often stemming from issues within the ABS system or CAN Bus network. A thorough diagnosis, starting with a comprehensive scan of all vehicle modules, is essential to accurately identify and resolve the root cause, ensuring the safety and proper functioning of your vehicle’s critical control systems.