AU
U010004

U010004 is an ISO 14229 CAN bus communication fault code — Seal U

Braking System

U010004 is an ISO 14229 CAN bus communication fault code.

It indicates the ESP (Electronic Stability Program) module detects a data validation error (CRC error), sequence error, or abnormal Data Length Code (DLC) when receiving a CAN data frame from the ECM (Engine Control Module, or the Vehicle Control Unit [VCU] / Motor Control Unit [MCU] in New Energy Vehicles).

This compromises data integrity.

Unlike U0100 (communication timeout), this fault indicates a physical connection exists but the data content is corrupted.

On BYD hybrid and battery electric platforms, this prevents the ESP from obtaining accurate motor torque, speed, or engine operating condition information.

This condition can trigger brake system downgrade mode (limiting ABS/ESP functions), disrupt energy recovery and brake coordination control, and in severe cases, cause power interruption or limp mode.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1CAN bus physical layer fault: Short circuit, open circuit, or short to power/ground in CAN-H and CAN-L lines, compromising signal integrity.
  • 2ECM/EMS module internal CAN transceiver damaged: An internal communication chip fault transmits error frames or corrupted data packets.
  • 3ESP module CAN receive circuit fault: Faulty ESP-side CAN controller or transceiver incorrectly identifies normal data as erroneous data.
  • 4Poor wiring harness connector contact: Oxidation, backed-out pins, or loose connections at the ECM or ESP module wiring harness connectors cause bit errors during signal transmission.
  • 5Electromagnetic interference (EMI): EMI generated by the high-voltage system (such as the motor, DC-DC converter, and air conditioning compressor) during operation couples into the CAN bus and disrupts data frames.
What To Do
  • 1
    Use the diagnostic tool to read all fault codes: confirm if U010004 is a current or history code, check if the ECM/VCU and other modules (TCU, BMS) have stored related communication fault codes, and analyze the freeze frame data captured when the fault occurred.
  • 2
    Measure CAN bus physical layer parameters: Use a multimeter to measure CAN-H (approx. 2.6-2.8V) and CAN-L (approx. 2.2-2.4V) voltage to ground. Measure terminating resistance (disconnect power and measure across the terminals; standard value is approx. 60Ω, single module is 120Ω). Use an oscilloscope to check the waveform for distortion, spikes, or abnormal bit width.
  • 3
    Inspect the wiring harness and connectors: focus on the ECM connector and ESP hydraulic unit connector in the engine compartment to check for oxidized, backed-out, or loose pins; inspect the wiring harness for signs of wear, crushing, or water ingress; measure the continuity and insulation of the CAN lines between the ECM and ESP.
  • 4
    Perform a network isolation test: Disconnect other modules on the CAN network one by one (such as the BMS, TCU, and air conditioning controller) and observe whether U010004 disappears to rule out other modules sending error frames that interfere with communication.
  • 5
    Module replacement verification: If wiring measurements are normal, swap the ECM (or VCU) or ESP module with a known-good unit to confirm if the fault follows the module. For hybrid models, also check the high-voltage system shielding layer ground.
  • 6
    Software and calibration check: Check ECM and ESP module software versions; upgrade or reflash if necessary. Perform CAN network configuration coding and ESP calibration procedure.
Real-World Cases
BYD DTC AI Analysis

BYD G6 ESP System Warning with Communication Fault

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Symptoms: Dashboard suddenly displayed 'Check ESP System' while driving. Warning returned after a few kilometres once codes were cleared. Diagnosis: ESP module stored historic DTC U010004 (EMS timeout). CAN bus voltage measured low 2.3V/high 2.7V and termination resistance 60Ω, both normal. Found slight oxidation on the ABS pump harness connector. Fix: Cleaning and tightening the ABS pump harness connector made no difference. Fault traced to internal communication failure in the engine control module. Replacing the engine ECU completely resolved the issue.
Original source ↗
BYD DTC AI Analysis

BYD S7 intermittent ESP warning light and multi-system communication fault

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Symptoms: ESP warning light illuminates intermittently with no discernible pattern. Diagnosis: ESP module logged DTC U010004 (EMS timeout, historical) and U012604 (SAS CAN communication timeout). TCU logged U1701 (communication failure with ECM). Engine system showed no DTCs. Root cause is intermittent interruption in CAN communication between ECM, ESP module, and TCU. Resolution: Inspection found loose CAN communication wiring harness connector between EMS and ESP module with pins slightly backed out. Repaired poor connection; fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Song petrol variant won't start with history DTCs

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Symptoms: 2016 BYD Song petrol manual fails to start, presenting as fuel starvation with stored U010004 fault in engine system. Diagnosis: Engine fires when spraying cleaner at throttle body, ruling out pure communication issue; live data shows abnormal fuel pressure, while U010004 indicates previous ECM communication fault. Solution: Low-pressure fuel pump malfunctioning, causing insufficient fuel supply; replaced low-pressure fuel pump to restore starting; checked ECM communication wiring to prevent U010004 from recurring.
Original source ↗
BYD DTC AI Analysis

BYD Song DM: Multiple system warnings and power interruption while driving

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Symptoms: Transmission fault, flashing P indicator, HDC warning and electronic park brake warning appeared suddenly while driving. Vehicle would not move. Diagnosis: No communication with VBU (Vehicle Control Unit). Front motor controller logged U014100 (lost communication with VBU). TCU and other modules reported communication faults with ECM (related U010004 faults). Diagnosed as CAN network failure causing ECM/VBU to lose communication with multiple systems. Resolution: Found poor contact in the CAN backbone wiring harness connector for the vehicle control unit. Reseated and secured the connector. All system communications returned to normal.
Original source ↗
Other Seal U Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.