DTC U01A887 Technical Diagnosis Explanation

Definition of Fault Depth

DTC U01A887 belongs to the U-Code network communication class faults, mainly referring to serious communication interruptions in the internal network link of the vehicle domain controller system. Specifically, this fault code indicates that data interaction between the Adaptive Cruise Control Unit and the Left Body Domain Controller can no longer maintain an effective connection. In modern domain architecture automotive electronic electrical systems, the Left Body Domain Controller acts as a critical node, responsible for coordinating sensors, actuators related to the left body side and status of the body domain network.

The ACC (Adaptive Cruise Control) system needs to receive real-time status parameters from the body in real time (such as vehicle speed reference, braking signals, steering angle input, etc.) to ensure the safety and logical closed-loop of longitudinal control. Once this communication link fails, it means the control unit cannot obtain necessary physical position feedback and speed signals, causing the system to enter a degraded protection mode, which is the "communication loss" described in the fault description.

Common Fault Symptoms

When DTC U01A887 is recorded and illuminated, owners may observe the following dashboard feedback and function abnormalities during driving:

• Partial ACC Function Failure: ACC function may fail to activate, or automatically exit after activation.
• Dashboard Warning Information Display: Related network communication fault indicator lights may turn on in the driver area, or text warnings (such as "System Fault", "Communication Loss").
• Longitudinal Control Downgrade: The vehicle loses automatic deceleration/acceleration ability based on leading vehicle distance, forced to switch to ordinary cruise control or full manual driving mode.
• Diagnostic Data Stream Anomalies: When reading via OBD interface, relevant network node status parameters display as "Lost" or "Timeout".

Core Fault Cause Analysis

According to the system architecture and DTC definition logic, the triggering of this fault code is usually caused by hardware or electronic issues in the following three dimensions:

• Power Supply Components (such as fuse failure): The protective components in the power supply circuit of the Left Body Domain Controller or ACC gateway are damaged, causing the control unit to fail to power up or have insufficient working voltage.
• Physical Connection Components (such as wiring harness or connector failure): The CAN bus wiring responsible for network transmission has open circuits, short circuits, loose pins, or oxidation corrosion in the wire bundle, destroying signal integrity.
• Controller Logic Components (such as Left Domain Controller failure, Pre-mounted Millimeter Wave Radar failure): Abnormal operation of the microprocessor inside the control unit, or hardware fault of associated upstream sensors (millimeter wave radar) causing it unable to respond to bus arbitration, thus being judged as lost communication by the upstream controller.

Technical Monitoring and Trigger Logic

The fault judgment algorithm of the control unit is based on strict time-series and signal status logic. Specific trigger conditions are as follows:

• Monitoring Target: The system monitors whether specific network messages (CAN Message) from the "Left Body Domain Controller" are transmitted normally within the expected period.
• Count Threshold: The system judgment requires satisfaction of one monitoring message lost continuously 10 times. This excludes false alarms caused by occasional signal jitter, ensuring the persistent characteristics of the fault.
• Supply Voltage Range: The electrical environment for fault judgment is limited to the controller working voltage range $9V$~$16V$. Exceeding this range will cause the system to prioritize cutting off or entering a sleep state, not executing such communication diagnosis.
• Timing Initialization Conditions: Fault recording starts effective monitoring only after being satisfied for full $3s$ after power-up initialization is completed, waiting for nodes to complete self-check and network handshake.
• Bus Status Protection: This code triggers only when the private CAN (Private CAN) has not entered Busoff status, preventing false reporting of communication loss in network dead or overload states.
• Vehicle Mode Restriction: Fault judgment is valid only when the factory mode is turned off and the vehicle is in normal driving or diagnostic mode, excluding interference signals during development debugging periods.