AU
U100004

DTC U100004 indicates the control module detects the CAN bus is in a "Bus Off" state — Atto 3

Braking System

DTC U100004 indicates the control module detects the CAN bus is in a "Bus Off" state.

In the BYD E5 battery electric vehicle, the ABS system reports this DTC.

It indicates the ABS control unit CAN controller automatically shuts down communication due to an error counter overflow (typically caused by continuous bus errors or interference) to protect the bus from a faulty node.

Consequently, the ABS module cannot exchange data with the vehicle network (including the gateway, VCU, and instrument cluster).

This disables functions such as the anti-lock braking system and electronic stability control (ESC) and illuminates multiple instrument panel warning lamps.

On some other models (such as the S6), U100004 may also indicate a transmission control module (TCU) communication fault, but the core mechanism remains a communication interruption between the control unit and the CAN bus.

The U100004 fault code on the Atto 3 indicates the ABS control module has detected a CAN bus “Bus Off” state, meaning it has stopped communicating with the vehicle network. Severity is medium because ABS/ESC functions may be disabled and warning lights can appear. Common causes include a faulty ABS CAN transceiver or CAN wiring shorted to power, ground, or another CAN line. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1A damaged internal CAN transceiver chip in the ABS control unit, or a cold solder joint, prevents sending or receiving CAN signals.
  • 2Chassis CAN bus lines (CAN-H/CAN-L) shorted to power, shorted to ground, or shorted together, causing bus blockage.
  • 3Abnormal ABS control unit power supply voltage (battery voltage too high/too low) or poor ground connection, causing abnormal communication module operation.
  • 4Gateway controller fault causes abnormal bus impedance matching or communication routing errors, indirectly triggering Bus Off at the ABS node.
  • 5CAN bus terminating resistor failure (open circuit or resistance drift) causing signal reflection, communication interference, and error counter accumulation.
What To Do
  • 1
    Use the BYD VDS diagnostic tool to read all fault codes. Confirm if U100004 is a current or history code, and check for other accompanying communication fault codes, such as U100308 (EPB CAN communication timeout).
  • 2
    Check the ABS control unit power supply: Measure the battery positive (B+) and ignition (IG) voltages. Voltages must be within 12V ± 0.5V. Ground resistance must be less than 1Ω.
  • 3
    Measure the CAN bus voltage at the OBD port or ABS connector: CAN-H is approximately 2.6-2.7 V, and CAN-L is approximately 2.3-2.4 V. Disconnect the power and measure the terminating resistance: approximately 60 Ω (two 120 Ω resistors in parallel).
  • 4
    Check the 32-pin ABS control unit connector (or corresponding connector) for water ingress, oxidation, or terminal back-out. Clean and apply conductive paste if necessary.
  • 5
    Perform a CAN bus short/open circuit test: Measure the resistance and voltage of CAN-H and CAN-L to ground and to the power supply, respectively. Verify no short circuits exist.
  • 6
    Attempt a power reset: Disconnect the 12V battery negative terminal for at least 5 minutes to clear the ABS control unit CAN controller error counter. Reconnect power and test.
  • 7
    If all above checks are normal but the fault persists, replace the ABS control unit assembly and perform online coding and ESC calibration.
Real-World Cases
BYD DTC AI Analysis

2018 BYD E5 ABS CAN Bus Off Fault

✓
Instrument cluster ABS and ESC warning lights stayed on constantly. VDS retrieved DTC U100004 (CAN bus off). Found pin 15 (CAN-H) at the ABS connector slightly oxidized. Cleaned and reconnected; fault persisted. ABS supply voltage measured normal. Disconnected the battery to reset; fault cleared. Recurred after one week of driving. Replaced the ABS control unit assembly and programmed it; fault completely resolved.
BYD DTC AI Analysis

BYD S6 U100004 Transmission Communication Fault

✓
The dashboard warning light came on. The scan tool read DTC U100004. Per technical documentation, this code relates to the transmission system on this model; the vehicle exhibited harsh shifting. Inspection revealed an internal chip failure in the Transmission Control Module (TCU) interrupting CAN communication. Replaced the TCU; fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Song DM Gateway Communication Fault (Reference Case)

✓
After starting the vehicle, multiple warning lights illuminated simultaneously (engine warning light, ABS light, airbag light, etc.) and some electronic functions failed. The scan tool could not establish communication with multiple control modules. U-category communication DTCs were retrieved. Checked the Gateway module power supply and CAN bus connections and found a loose Gateway connector causing the communication interruption. Reconnected the Gateway module connector, cleaned the terminal oxidation, and resolved the fault.
Original source ↗
BYD DTC AI Analysis

BYD S6 instrument panel communication fault (U100004-type symptoms)

✓
The owner reported inoperative steering wheel buttons, inability to set cruise control, and abnormal instrument cluster displays. Technicians replaced the steering wheel cruise control switch assembly, instrument panel relay assembly, and instrument cluster assembly, but the fault remained. Troubleshooting revealed poor contact in the CAN bus lines between the instrument cluster, BCM, and gateway. Repairing the wiring harness connector resolved the issue. This case demonstrates how CAN bus communication faults can affect multiple modules.
Original source ↗
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 overview →
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.