The diagnostic trouble code (DTC) U0139, "Lost Communication With Body Control Module (BCM)," is a communication error code indicating that the vehicle's computer network has lost communication with the Body Control Module (BCM). This can manifest in a variety of electrical malfunctions throughout the vehicle since the BCM controls numerous systems. Understanding the causes, symptoms, and troubleshooting steps for U0139 is crucial for effectively diagnosing and resolving this issue.
The U0139 code is most commonly seen in vehicles with a Controller Area Network (CAN) bus system, which allows various electronic control units (ECUs) to communicate with each other. When the BCM becomes unresponsive or the communication link is disrupted, the other modules detect this loss of communication and store the U0139 code.
U0139: Key Information
| Topic | Description | Possible Causes |
|---|---|---|
| Definition | Lost communication with the Body Control Module (BCM). Indicates a failure in the CAN bus network preventing communication between the BCM and other modules. | Faulty BCM, CAN bus wiring issues (shorts, opens, corrosion), CAN bus resistor problems, ECM/PCM failure, Power or ground issues to the BCM, Software glitches in the BCM or other modules, Corroded or loose connectors, Blown fuses related to the BCM or CAN bus network, Interference from aftermarket electronics, Damaged or improperly installed aftermarket accessories. |
| Common Symptoms | Check Engine Light (CEL) illumination, Malfunctioning electrical components (lights, wipers, power windows, door locks, etc.), Inability to start the vehicle, Erratic gauge behavior, Transmission problems, ABS or traction control malfunctions, Airbag warning light. | |
| Diagnostic Steps | Verify the code with a scanner, Inspect CAN bus wiring for damage, Check BCM power and ground connections, Test CAN bus voltage and resistance, Scan other modules for related codes, Inspect connectors for corrosion, Update BCM software (if available), Test the BCM using a professional scan tool, Check fuses and relays related to the BCM, Disconnect aftermarket accessories to rule out interference. |
Detailed Explanations
Definition: The U0139 code signifies a complete loss of communication between the Body Control Module (BCM) and other modules on the vehicle's Controller Area Network (CAN) bus. The BCM acts as a central hub for controlling many of the vehicle's electrical systems, so its absence disrupts the normal operation of these systems. The code indicates a malfunction within the network itself, preventing data exchange.
Common Symptoms: The symptoms associated with a U0139 code can be quite varied due to the BCM's wide range of responsibilities. The Check Engine Light (CEL) is almost always illuminated. You might experience issues with various electrical components like lights, wipers, power windows, and door locks. In some cases, the vehicle may not start. Erratic gauge behavior, transmission problems, ABS/traction control malfunctions, and even airbag warning lights can also occur. These symptoms highlight the interconnectedness of the vehicle's systems and the BCM's role in managing them.
Diagnostic Steps: Diagnosing a U0139 code requires a systematic approach. First, verify the code with a scan tool to confirm its presence. Then, inspect the CAN bus wiring for any signs of damage, such as shorts, opens, or corrosion. Check the BCM's power and ground connections to ensure they are secure and providing the correct voltage. Test the CAN bus voltage and resistance to verify the integrity of the network. Scan other modules for related codes that might provide further clues. Carefully inspect connectors for corrosion or loose connections. If available, update the BCM software to the latest version. Test the BCM using a professional-grade scan tool to assess its functionality. Check all fuses and relays related to the BCM and CAN bus network. Finally, disconnect any aftermarket accessories to rule out potential interference.
Frequently Asked Questions
What does the U0139 code mean? It means there's a communication failure between the Body Control Module (BCM) and other modules on the vehicle's network. This indicates a problem with the CAN bus network or the BCM itself.
Can I drive my car with a U0139 code? While you might be able to drive, it's not recommended. Because the BCM controls many vital systems, you could experience unexpected electrical malfunctions or safety-related issues.
What causes the U0139 code? Possible causes include a faulty BCM, wiring issues in the CAN bus network, power or ground problems to the BCM, or software glitches. Corrosion and loose connections are also common culprits.
How do I fix the U0139 code? Diagnosis and repair typically involve checking the CAN bus wiring, inspecting BCM connections, testing the BCM itself, and potentially replacing the BCM or repairing wiring. Professional help is often required.
Is the U0139 code serious? Yes, the U0139 code can be serious because it indicates a disruption in the vehicle's communication network, potentially affecting critical systems and safety features.
Can a bad battery cause a U0139 code? Yes, a weak or failing battery can sometimes cause voltage fluctuations that disrupt communication on the CAN bus, leading to a U0139 code.
Common Causes Explained in Detail
Faulty BCM: The BCM itself can fail due to internal component damage or software corruption. This is often the most expensive and complex cause to address, requiring replacement of the BCM and subsequent programming. A BCM failure can be triggered by voltage spikes, physical damage, or simply the wear and tear of components over time.
CAN Bus Wiring Issues (Shorts, Opens, Corrosion): The CAN bus network relies on a complex network of wires to transmit data between modules. Damage to these wires, such as shorts to ground or power, open circuits, or corrosion, can disrupt communication and trigger the U0139 code. Rodent damage, chafing against sharp edges, and exposure to moisture are common causes of wiring problems.
CAN Bus Resistor Problems: The CAN bus network utilizes terminating resistors at each end of the bus to prevent signal reflections and ensure proper communication. If these resistors fail or are disconnected, it can disrupt the network's impedance and cause communication errors. The resistors are typically located within the ECUs at the ends of the CAN bus.
ECM/PCM Failure: While the U0139 code specifically points to the BCM, a malfunctioning Engine Control Module (ECM) or Powertrain Control Module (PCM) can sometimes interfere with the CAN bus communication and indirectly trigger the code. The ECM/PCM is a critical node on the CAN bus, and its malfunctions can have widespread effects on the network.
Power or Ground Issues to the BCM: The BCM requires a stable and consistent power supply and a reliable ground connection to function correctly. Voltage drops, loose ground connections, or corroded terminals can disrupt the BCM's operation and lead to a communication failure. Always check the battery voltage and the integrity of the ground connections.
Software Glitches in the BCM or other modules: Software errors or glitches in the BCM or other modules on the CAN bus can sometimes disrupt communication and trigger the U0139 code. A software update or reprogramming of the BCM might be necessary to resolve this issue.
Corroded or Loose Connectors: Corrosion or loose connections in the wiring harness connecting the BCM to the CAN bus can interrupt the flow of data and cause the U0139 code. Thoroughly inspect all connectors for any signs of corrosion or damage and ensure they are securely connected.
Blown Fuses Related to the BCM or CAN Bus Network: A blown fuse in the BCM's power circuit or a fuse protecting the CAN bus network can obviously cause a communication failure. Always check all relevant fuses before proceeding with more complex diagnostics.
Interference from Aftermarket Electronics: Aftermarket accessories, such as alarms, remote starters, or audio systems, can sometimes interfere with the CAN bus network if they are improperly installed or malfunctioning. Disconnecting these accessories can help determine if they are the source of the problem.
Damaged or Improperly Installed Aftermarket Accessories: Even properly installed aftermarket accessories can cause issues if they are drawing excessive current or generating electrical noise on the CAN bus. Ensure that all aftermarket accessories are properly installed and functioning correctly.
Recommended Actions
If you encounter a U0139 code, it is highly recommended to consult a qualified automotive technician with experience in diagnosing CAN bus communication problems. They have the necessary tools and knowledge to accurately pinpoint the root cause of the issue and perform the appropriate repairs. Attempting to diagnose and repair the problem yourself without the proper equipment and expertise could potentially lead to further damage to the vehicle's electrical system.
Conclusion
The U0139 code, "Lost Communication With Body Control Module (BCM)," indicates a critical communication failure within the vehicle's network, requiring careful diagnosis and repair. Addressing this code promptly is essential to restore proper functionality to the vehicle's electrical systems and ensure safe operation.