U0063 Vehicle Communication Bus D (-) shorted to Bus D (+)

Introduction:

The U0063 diagnostic trouble code (DTC) indicates a serious fault within the vehicle's communication network. Specifically, it signifies that the Bus D (-) circuit is shorted to the Bus D (+) circuit. This short circuit disrupts the normal communication between electronic control units (ECUs) on the Bus D network, potentially leading to a cascade of issues affecting various vehicle systems. Understanding the cause and effect of this DTC is crucial for effective diagnosis and repair.

Table: U0063 Vehicle Communication Bus D (-) Shorted to Bus D (+)

Topic	Description	Possible Causes
Definition	Bus D (-) shorted to Bus D (+) implies an unintended electrical connection between the negative (D-) and positive (D+) wires of the Bus D communication network. This creates a direct path for current, bypassing the intended circuitry and disrupting the differential signaling that is key to CAN bus function.	Damaged Wiring Harness: This is the most common cause. Abrasion, cuts, or rodent damage can expose the wires, leading to a short. Faulty Connectors: Corrosion, loose pins, or physical damage to connectors can create a short. Internal Short in an ECU: A malfunctioning ECU connected to the Bus D network can internally short the D- and D+ lines. Aftermarket Accessories: Improperly installed accessories can interfere with the CAN bus wiring. Water Intrusion: Water can cause corrosion and short circuits within the wiring harness or connectors.
Symptoms	The symptoms of U0063 can vary depending on which ECUs are on Bus D and their functions. Common symptoms include: Loss of communication with specific ECUs, Malfunction Indicator Lamp (MIL) illumination, Erratic behavior of affected systems (e.g., ABS, transmission, stability control), Vehicle may not start, Warning lights on the dashboard, Reduced performance, Data logging errors.
Affected Systems	The specific systems affected by U0063 depend on which ECUs are connected to the Bus D network. Examples include: Anti-lock Braking System (ABS), Transmission Control Module (TCM), Electronic Stability Control (ESC), Body Control Module (BCM), Instrument Cluster, Engine Control Module (ECM) (depending on vehicle architecture), Power Steering Control Module (PSCM).
Diagnostic Procedure
Common Tools

Detailed Explanations:

Definition:

The U0063 code indicates that the intended voltage difference between the CAN Bus D+ and CAN Bus D- wires has been disrupted due to a short circuit. In a properly functioning CAN bus system, data is transmitted as a difference in voltage between these two wires. When they are shorted together, this voltage difference is eliminated, preventing proper communication.

Symptoms:

The wide range of possible symptoms is due to the distributed nature of CAN bus systems. Because many ECUs rely on information transmitted over the bus, a communication failure can affect numerous vehicle functions. The severity and type of symptoms will depend on which ECUs are on Bus D, their roles in the vehicle, and how critical the information they share is. For example, if the ABS ECU is on Bus D and cannot communicate, the ABS system will likely be disabled, resulting in a warning light and potential loss of ABS functionality. Similarly, transmission shifting problems can occur if the TCM cannot communicate with the ECM or other relevant modules.

Affected Systems:

Identifying the affected systems is crucial for narrowing down the source of the problem. By understanding which modules communicate on the Bus D network, you can focus your diagnostic efforts on those specific components and their associated wiring. Vehicle-specific wiring diagrams are essential for determining the exact configuration of the CAN bus network.

Diagnostic Procedure:

The diagnostic procedure is a systematic approach to isolating the short circuit. It begins with a visual inspection to identify obvious damage and progresses to more complex testing methods. Safety is paramount when working with electrical systems. Always disconnect the battery before performing any electrical testing to prevent accidental shocks or damage to the vehicle's electronics.

Visual Inspection: This is the first and often most revealing step. Pay close attention to areas where the wiring harness is routed near sharp edges, exhaust components, or moving parts, as these are common locations for damage. Look for signs of rodent activity, such as chewed wires or nesting material. Inspect connectors for corrosion, bent pins, or loose connections.
Continuity Testing: This test verifies whether there is a direct electrical connection between the Bus D (+) and Bus D (-) wires. With the battery disconnected, use a multimeter set to the continuity setting. Place one probe on the Bus D (+) wire and the other probe on the Bus D (-) wire. If the multimeter indicates continuity (typically with a beep or a reading of 0 ohms), it confirms a short circuit between the two wires.
Resistance Testing: This test measures the resistance between the Bus D (+) and Bus D (-) wires. A low resistance reading (close to 0 ohms) indicates a short circuit. This test can be helpful in identifying a partial short, where the connection is not a direct short but still allows excessive current to flow.
Voltage Testing: This test measures the voltage on the Bus D (+) and Bus D (-) wires with the ignition on (but engine off). A normal CAN bus system will have specific voltage levels on each wire, typically around 2.5 volts when idle, with variations during data transmission. If the voltage on both wires is the same (e.g., close to battery voltage or ground), it indicates a short circuit.
CAN Bus Analyzer: A CAN bus analyzer is a specialized tool that allows you to monitor the communication signals on the CAN bus network. It can display the data being transmitted, identify error messages, and help pinpoint the source of communication problems. This tool is particularly useful for diagnosing intermittent faults or for troubleshooting complex CAN bus issues.
ECU Isolation: This method involves disconnecting ECUs one at a time to see if the short circuit disappears. Disconnecting an ECU removes it from the Bus D network and eliminates it as a potential source of the short. If disconnecting a particular ECU resolves the short, it indicates that the ECU itself or the wiring leading to it is the cause of the problem.
Consult the Vehicle Service Manual: The vehicle service manual is an invaluable resource for diagnosing CAN bus problems. It provides detailed wiring diagrams, component locations, and specific diagnostic procedures for the vehicle. Always refer to the service manual for accurate information and guidance.

Common Tools:

Having the right tools is essential for efficient and accurate diagnosis.

Multimeter: A multimeter is a versatile tool used for measuring voltage, current, resistance, and continuity. It is an indispensable tool for electrical troubleshooting.
CAN Bus Analyzer: A CAN bus analyzer is a specialized tool used for monitoring and analyzing CAN bus communication. It can display the data being transmitted, identify error messages, and help pinpoint the source of communication problems.
Wiring Diagrams: Vehicle-specific wiring diagrams are essential for identifying the location of components and wire routing. They provide a visual representation of the electrical circuits in the vehicle.
Diagnostic Scan Tool: A diagnostic scan tool is used to read and clear DTCs. It can also provide live data from various sensors and modules, which can be helpful in diagnosing CAN bus problems.
Basic Hand Tools: Basic hand tools, such as screwdrivers, pliers, and wire strippers, are necessary for accessing and repairing wiring and connectors.

Frequently Asked Questions:

What is a CAN bus? The Controller Area Network (CAN) bus is a robust communication network used in vehicles to allow different electronic control units (ECUs) to communicate with each other.
What does "shorted to" mean? "Shorted to" means that two or more wires that should be electrically isolated are unintentionally connected, creating a low-resistance path for current.
Can I fix this myself? While some basic troubleshooting can be done with a multimeter and visual inspection, diagnosing and repairing CAN bus problems often requires specialized tools and knowledge. Consider consulting a qualified technician.
How much will it cost to fix? The cost of repair depends on the location and severity of the short circuit, as well as the labor rates in your area. It can range from a few hundred dollars to several thousand dollars.
Is it safe to drive with this code? Driving with a U0063 code may be unsafe, as it can affect critical vehicle systems such as ABS, transmission, and stability control. It is recommended to have the problem diagnosed and repaired as soon as possible.

Conclusion:

The U0063 DTC indicates a critical fault in the vehicle's communication network. A systematic diagnostic approach, using the appropriate tools and resources, is essential for identifying and repairing the short circuit. Prioritize safety and consult a qualified technician if you lack the necessary expertise.