U018087 - U018087 Communication Lost with High Side Drive

U018087 and High Side Drive Communication Lost Fault Code Technical Explanation

Detailed Fault Definition

U018087 and High Side Drive Communication Lost (Lost Communication with High Side Drive) is a critical general diagnostic code involving the high-voltage drive system in the vehicle's power system. In new energy architecture, this fault code explicitly indicates that the vehicle central control unit (BMS or VCU) cannot establish an effective data interaction link with the high-side drive module via the SPI (Serial Peripheral Interface) bus.

In the high-side drive system, the high-side drive controller is usually responsible for managing the switching state of power semiconductors and the energy flow direction to/from the motor/charger. The core significance of this fault code lies in breakage or data frame check failure in the physical communication link between the control unit and the drive module. Since SPI communication carries tasks such as real-time feedback of motor position, rotation speed, and high-voltage loop status, once communication is lost, the system cannot confirm the electrical safety status on the high-side drive side, thereby triggering protection logic to isolate the high-voltage system. This code typically belongs to the U-class network fault code category, involving cross-module information synchronization failure between the power domain and battery management domain.

Common Fault Symptoms

When the vehicle monitoring system determines that the U018087 fault code is triggered, the vehicle onboard diagnostic system (OBD) will immediately execute safety restriction strategies, specific driving experience and instrument feedback perceivable by the driver are as follows:

• Dashboard Alarm: The vehicle information center clearly displays "Power System Fault" or equivalent red warning icons.
• High Voltage Output Restricted: The system will immediately prohibit external discharge functions (such as V2L power taking mode), and also prohibit charging of the power battery, preventing forced energy charging without drive control feedback.
• Driving Interruption: The vehicle may enter "limp mode" or directly cut off power output, causing the vehicle to be unable to continue driving or only able to maintain very low energy consumption coasting.

Core Fault Cause Analysis

Regarding the root cause troubleshooting for the U018087 fault code, analysis based on system architecture from the following three technical dimensions is required:

• Hardware Component Failure: Mainly involves SPI communication chip failure inside the high-side drive module, or damage to the communication interface circuit integrated inside the power battery pack. According to original data description, "Power Battery Pack Internal Fault" is the primary suspect object, meaning internal high-side drive communication lines or main control units of the battery pack may have physical damage.
• Line and Connector Abnormality: SPI communication lines between the high-side drive module and control unit (containing clock lines, data lines) may appear open, short-circuited or impedance abnormal; simultaneously, connector oxidation, pin bending or locking mechanism not engaged will also lead to unstable signal transmission, thereby causing communication interruption.
• Controller Logic Operation Abnormality: Control strategies responsible for monitoring communication status may exist software misjudgment, or under specific network load conditions, high-side drive controller communication response timeout, leading to the main controller incorrectly recording this fault code.

Technical Monitoring and Trigger Logic

System judgment of U018087 strictly follows specific electrical logic and timing conditions to ensure lowest false report rate:

• Monitoring Target: System real-time monitors integrity of SPI data frames returned by high-side drive module, clock frequency (SCL) synchronization and whether signal voltage level is within effective threshold range.
• Trigger Condition:
  • Vehicle Power-On State: Fault only triggers during system self-check stage after ignition switch ON or high-voltage relay closed, and during running period.
  • Dynamic Monitoring Logic: When drive motor is working, system polls and monitors data exchange frequency on SPI bus.
• Fault Judgment Threshold: When system does not receive effective response from high-side drive within a continuous time window (such as communication heartbeat interval), or received data checksum bits (CRC) errors exceed preset tolerance, fault code record is generated. This logic ensures that under vehicle power-on state, after system detects substantive fault in High Side Drive SPI communication, the DTC will be formally marked.