The diagnostic trouble code (DTC) U0138, "Lost Communication With Rear Body Control Module," indicates a communication failure between the vehicle's computer and the rear body control module (RBCM). This module is responsible for controlling various functions at the rear of the vehicle, such as lights, power windows, and door locks. Understanding the causes, symptoms, and potential solutions for U0138 is crucial for diagnosing and resolving issues related to these systems.
U0138: A Comprehensive Overview
| Category | Description | Possible Causes |
|---|---|---|
| Definition | Lost Communication With Rear Body Control Module | CAN Bus Issues, RBCM Failure, Wiring Problems, Power Supply Issues, Software Glitches |
| Symptoms | Malfunctioning Rear Lights, Inoperative Power Windows (Rear), Non-Functional Door Locks (Rear), Battery Drain, DTC U0138 Present | Damaged Wiring, Corroded Connectors, Faulty RBCM, Low Battery Voltage, Corrupted Software |
| Diagnostic Steps | CAN Bus Inspection, Power & Ground Check, RBCM Testing, Wiring Harness Inspection, Software Update | Scan Tool, Multimeter, Wiring Diagram, Visual Inspection, Software Diagnostic Tools |
Detailed Explanations
CAN Bus Issues: The Controller Area Network (CAN) bus is a communication network that allows various electronic control units (ECUs) in a vehicle to communicate with each other. If there is a problem with the CAN bus, such as a broken wire or a faulty terminator resistor, the RBCM may not be able to communicate with other modules, resulting in the U0138 code. Proper CAN bus voltage and resistance are critical for correct communication.
RBCM Failure: The Rear Body Control Module (RBCM) itself may have failed due to internal component damage, voltage spikes, or other electrical issues. A faulty RBCM will not be able to perform its functions and will likely trigger the U0138 code. In some cases, the failure can be intermittent, making diagnosis more challenging.
Wiring Problems: Issues with the wiring harness connecting the RBCM to the CAN bus or the power supply can cause communication problems. This includes damaged wires, corroded connectors, and loose connections. A thorough inspection of the wiring harness is essential to identify and repair any such issues.
Power Supply Issues: The RBCM requires a stable power supply to operate correctly. If the voltage supplied to the module is too low or unstable, it may not be able to communicate with other modules. Low battery voltage, a faulty alternator, or a bad ground connection can all contribute to power supply issues.
Software Glitches: In some cases, the RBCM's software may be corrupted or have a glitch that prevents it from communicating properly. Reprogramming or updating the RBCM's software may resolve the issue. This is often a last resort after other potential causes have been ruled out.
Malfunctioning Rear Lights: The RBCM often controls the rear lights, including the brake lights, turn signals, and taillights. If the RBCM is not functioning correctly due to a communication problem, these lights may not work properly. This can include the lights not turning on, staying on constantly, or flickering.
Inoperative Power Windows (Rear): Many vehicles have power windows in the rear doors that are controlled by the RBCM. If the RBCM cannot communicate with the main computer, the rear power windows may not function at all. This symptom specifically points towards an issue with the rear portion of the vehicle's electrical system.
Non-Functional Door Locks (Rear): The RBCM may also control the rear door locks. If the module is not communicating, the rear door locks may not respond to the key fob or the interior door lock switch. This can be a security concern as well as an inconvenience.
Battery Drain: A malfunctioning RBCM can sometimes cause a parasitic battery drain, meaning that it continues to draw power from the battery even when the vehicle is turned off. This can lead to a dead battery if the vehicle is left unused for an extended period.
DTC U0138 Present: The presence of the U0138 diagnostic trouble code is the primary indicator of a communication problem with the RBCM. The code is stored in the vehicle's computer and can be retrieved using a scan tool.
Damaged Wiring: Physical damage to the wiring harness, such as cuts, abrasions, or exposure to the elements, can disrupt the electrical signals and cause communication problems. Rodent damage is a common cause of wiring damage.
Corroded Connectors: Corrosion on the electrical connectors can impede the flow of electricity and cause intermittent or complete communication failures. Corrosion is often caused by moisture and can be exacerbated by road salt.
Faulty RBCM: As mentioned earlier, the RBCM itself can be faulty. Internal component failures can prevent the module from functioning correctly.
Low Battery Voltage: Insufficient battery voltage can prevent the RBCM from operating properly. A weak or failing battery may not be able to provide the necessary power to the module.
Corrupted Software: The software that controls the RBCM can become corrupted due to various factors, such as voltage spikes or programming errors. This can lead to communication problems and other malfunctions.
CAN Bus Inspection: This involves checking the CAN bus wires for any damage or breaks, as well as verifying the proper voltage and resistance. A multimeter is used to measure the voltage and resistance of the CAN bus.
Power & Ground Check: This involves verifying that the RBCM is receiving the correct voltage and that the ground connection is secure. A multimeter is used to check the voltage and continuity of the power and ground circuits.
RBCM Testing: The RBCM can be tested using a scan tool or specialized diagnostic equipment to check its functionality and communication capabilities. Some scan tools can perform specific tests on the RBCM to diagnose problems.
Wiring Harness Inspection: A thorough visual inspection of the wiring harness is necessary to identify any damaged wires, corroded connectors, or loose connections. Pay close attention to areas where the wiring harness is exposed to the elements or where it may be subject to abrasion.
Software Update: If the RBCM's software is suspected to be corrupted, it may be necessary to update or reprogram the module using specialized software and equipment. This should only be done by a qualified technician.
Scan Tool: A scan tool is a diagnostic tool that can be used to retrieve diagnostic trouble codes (DTCs) from the vehicle's computer. It can also be used to monitor various parameters and perform diagnostic tests.
Multimeter: A multimeter is an electrical testing tool that can be used to measure voltage, current, and resistance. It is essential for diagnosing electrical problems in vehicles.
Wiring Diagram: A wiring diagram is a schematic that shows the electrical connections between various components in the vehicle. It is essential for tracing circuits and identifying wiring problems.
Visual Inspection: A visual inspection involves carefully examining the wiring harness, connectors, and other components for any signs of damage or corrosion. This is often the first step in diagnosing electrical problems.
Software Diagnostic Tools: These are specialized software programs that can be used to diagnose problems with the RBCM's software. They can also be used to update or reprogram the module.
Frequently Asked Questions
What does U0138 mean? It indicates a loss of communication with the Rear Body Control Module (RBCM). The vehicle's computer can't communicate with the module controlling rear functions.
What are the common symptoms of U0138? Malfunctioning rear lights, inoperative rear power windows, and non-functional rear door locks are common. A battery drain may also occur.
Can I fix U0138 myself? Basic checks like wiring inspection are possible, but professional diagnosis and repair are often needed. Specialized tools and knowledge are usually required.
What causes the U0138 code? CAN bus issues, a faulty RBCM, wiring problems, power supply issues, or software glitches. These problems prevent communication with the RBCM.
How do I clear the U0138 code? A scan tool is needed to clear the code after the underlying problem has been fixed. Simply clearing the code without addressing the cause will result in its return.
Conclusion
The U0138 code points to a communication breakdown with the rear body control module, potentially impacting several vehicle functions. Thorough diagnosis, starting with a visual inspection and progressing to more advanced testing, is crucial to pinpoint the root cause. Consulting a qualified mechanic is recommended if the issue persists or requires specialized tools and expertise.