U0035 Vehicle Communication Bus A (-) High

Introduction:

The U0035 diagnostic trouble code (DTC) indicates a problem with the "Vehicle Communication Bus A (-) High" circuit in a vehicle's Controller Area Network (CAN) bus system. This CAN bus is a critical communication network that allows various electronic control units (ECUs) within the vehicle to share information, enabling coordinated operation of systems like the engine, transmission, brakes, and body controls. A "High" fault suggests the voltage on the CAN bus (-) wire is exceeding the expected range, disrupting communication and potentially causing a variety of symptoms.

Comprehensive Table: U0035 Vehicle Communication Bus A (-) High

Detailed Explanations:

Definition: The U0035 DTC signifies that the voltage on the CAN Low (-) wire of the "Vehicle Communication Bus A" is higher than the expected operating range. This abnormal voltage level interferes with the proper communication of data between different ECUs on that bus.

CAN Bus Basics: The Controller Area Network (CAN) bus is a robust communication system used in modern vehicles. Instead of relying on point-to-point wiring between each ECU, the CAN bus allows multiple ECUs to share data over a single pair of wires. This reduces wiring complexity and improves efficiency. It uses differential signaling with two wires, CAN High (+) and CAN Low (-), to transmit data.

Typical Voltage Ranges: In a typical CAN bus system, the CAN High (+) wire rests around 2.5V and rises to approximately 3.5V during data transmission. Conversely, the CAN Low (-) wire rests around 2.5V and drops to about 1.5V during communication. The difference in voltage between CAN High and CAN Low represents the data being transmitted. A U0035 code indicates that the CAN Low (-) voltage is abnormally high, remaining above the expected range even when the bus is at rest or transmitting data.

Symptoms: The symptoms of a U0035 code can vary widely depending on the specific vehicle and the modules connected to the affected CAN bus. Common symptoms include the illumination of the Malfunction Indicator Lamp (MIL), erratic behavior of vehicle systems such as transmission shifting problems or ABS malfunctions, a complete loss of communication with one or more ECUs, and in some cases, the vehicle may not start. Instrument cluster errors or warning lights are also frequently observed.

Common Causes: Several factors can contribute to a U0035 DTC. A short to voltage on the CAN Low (-) wire is a common culprit, where the wire inadvertently connects to a voltage source, raising its voltage level. A faulty ECU sending an incorrect voltage on the CAN Low (-) line can also trigger the code. Damage or corrosion to the wiring harness, especially affecting the CAN bus wires, can disrupt communication. While less common for a "High" fault, a CAN bus terminator resistor failure can sometimes contribute to the problem. Finally, aftermarket accessories that are improperly connected to the CAN bus can introduce interference and cause voltage imbalances.

Diagnostic Steps: Diagnosing a U0035 code requires a systematic approach. Start by using an OBDII scanner to check for other related DTCs that might provide clues. Then, thoroughly inspect the CAN bus wiring harness for any signs of damage, corrosion, or shorts. Use a multimeter to check the CAN bus voltage levels at various ECUs and at the DLC (Data Link Connector/OBDII port) to identify the source of the high voltage. Disconnecting ECUs one at a time can help isolate the faulty module. Verify the CAN bus terminator resistor values to ensure they are within the specified range (typically 120 ohms). Finally, consult vehicle-specific wiring diagrams to understand the CAN bus routing and component locations.

Affected Systems: The systems affected by a U0035 code depend on which ECUs are connected to the "Vehicle Communication Bus A." Commonly affected systems include the Engine Control Module (ECM), Transmission Control Module (TCM), Anti-lock Braking System (ABS), Body Control Module (BCM), and the Instrument Cluster. The malfunction of these systems can lead to a variety of drivability and safety issues.

Tools Required: A successful diagnosis and repair of a U0035 code requires several tools. An OBDII scanner with CAN bus diagnostic capabilities is essential for reading DTCs and monitoring CAN bus data. A multimeter is needed to measure voltage, resistance, and continuity. Wiring diagrams specific to the vehicle are crucial for understanding the CAN bus layout and component locations. A CAN bus analyzer is an optional but highly recommended tool for advanced diagnostics, allowing for real-time monitoring of CAN bus traffic and identification of communication errors.

Potential Fixes: The corrective action for a U0035 code depends on the underlying cause. Repairing or replacing damaged CAN bus wiring is often necessary to eliminate shorts or open circuits. A faulty ECU may need to be replaced if it is sending incorrect voltage on the CAN Low (-) line. Correctly terminating the CAN bus with 120-ohm resistors at each end ensures proper impedance matching. Removing or properly installing aftermarket accessories connected to the CAN bus can eliminate interference. Repairing corroded connectors can restore proper electrical connections.

Related DTCs: A U0035 code is often accompanied by other DTCs related to CAN bus communication failures. Common related codes include U0001 (High Speed CAN Communication Bus), U0002 (High Speed CAN Communication Bus Performance), U0100 (Lost Communication With ECM/PCM 'A'), U0101 (Lost Communication With TCM), U0121 (Lost Communication With ABS Control Module), U0155 (Lost Communication With Instrument Panel Cluster (IPC) Control Module), and other U-codes related to communication issues with specific modules.

Bus A vs. Bus B, etc.: Modern vehicles often employ multiple CAN buses to segregate communication based on priority and function. "Bus A" is typically the primary bus responsible for critical powertrain and safety systems. Other buses, such as "Bus B" or "Bus C," might handle less critical functions like infotainment or comfort features. Understanding which bus is affected is crucial for accurate diagnosis.

Importance of Wiring Diagrams: Vehicle-specific wiring diagrams are absolutely essential for diagnosing CAN bus problems. They provide detailed information about the ECUs connected to "Vehicle Communication Bus A," the routing of the wiring harness, the location of connectors, and the placement of terminator resistors. Without these diagrams, it is difficult to trace the CAN bus and identify potential faults.

Impact of Aftermarket Accessories: Improperly installed aftermarket accessories, especially those that tap into the CAN bus for power or data, can introduce significant problems. These accessories can create noise, shorts, or voltage imbalances on the CAN bus, leading to communication errors and DTCs like U0035. It's crucial to ensure that any aftermarket accessories are installed correctly and do not interfere with the CAN bus system.

Terminator Resistors: Terminator resistors are critical components of the CAN bus system. They are typically located at the physical ends of the bus and are designed to provide proper impedance matching. This prevents signal reflections and ensures reliable communication. A faulty terminator resistor can disrupt CAN bus communication and trigger DTCs. The resistance should be approximately 120 ohms.

ECU Identification: Each ECU on the CAN bus is assigned a unique identifier called a CAN ID. This allows ECUs to communicate specific data to other modules on the bus. A fault within an ECU can lead to the transmission of incorrect CAN IDs or corrupted data, which can cause communication errors and system malfunctions.

Data Link Connector (DLC): The Data Link Connector (DLC), also known as the OBDII port, is the interface where scan tools connect to access vehicle data. CAN bus voltages can be measured at specific pins on the DLC, providing a convenient point for initial diagnostics.

Frequently Asked Questions:

• What does U0035 code mean? The U0035 code signifies that the CAN Low (-) wire of the "Vehicle Communication Bus A" has a voltage that is too high. This interferes with proper communication between ECUs.

• Can I drive with a U0035 code? Driving with a U0035 code is not recommended, as it can cause erratic vehicle behavior and potentially compromise safety systems. It is best to address the issue as soon as possible.

• How do I fix a U0035 code? Fixing a U0035 code involves diagnosing the underlying cause, which could be a short to voltage, a faulty ECU, wiring damage, or a terminator resistor issue. Repairing or replacing the affected components will resolve the problem.

• Is a U0035 code serious? Yes, a U0035 code can be serious because it indicates a problem with a critical communication network in the vehicle. This can affect various systems, including the engine, transmission, and brakes.

• What is the CAN bus? The CAN bus (Controller Area Network) is a communication network that allows different electronic control units (ECUs) in a vehicle to share information. This allows for coordinated operation of various systems.

Conclusion:

The U0035 DTC indicates a high voltage issue on the CAN Low (-) wire of the vehicle's primary communication bus, potentially disrupting the operation of several vital vehicle systems. Systematic diagnosis utilizing wiring diagrams and proper testing equipment is crucial to identify and resolve the root cause, ensuring the vehicle's systems can communicate effectively and function as intended.