AU
U100004

DTC U100004 indicates the control module detects the CAN bus is in a "Bus Off" state — Atto 8

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.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 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 circuit (CAN-H/CAN-L) shorted to power, shorted to ground, or shorted together, causing bus blockage.
  • 3Abnormal power supply voltage to the ABS control unit (battery voltage too high/too low) or poor ground connection, causing abnormal communication module operation.
  • 4A gateway controller fault causes abnormal bus impedance matching or a communication routing error, indirectly triggering an ABS node Bus Off.
  • 5CAN bus terminating resistor failure (open circuit or resistance drift) causes signal reflection, communication interference, and error counter accumulation.
What To Do
  • 1
    Use the BYD VDS diagnostic tool to read all fault codes. Confirm whether U100004 is a current or history code, and check for accompanying communication fault codes such as U100308 (EPB CAN communication timeout).
  • 2
    Check the ABS control unit power supply: Measure the constant power (B+) and ignition power (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 connector or ABS plug: CAN-H is approximately 2.6-2.7V, and CAN-L is approximately 2.3-2.4V. Disconnect power and measure the terminal resistance; it should be approximately 60Ω (two 120Ω resistors in parallel).
  • 4
    Check the ABS control unit 32-pin connector (or corresponding connector) for water ingress, oxidation, or backed-out pins. Clean and apply conductive grease 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 power respectively to confirm no short circuit exists.
  • 6
    Perform a power reset: Disconnect the battery negative terminal for more than 5 minutes to clear the ABS control unit CAN controller error counter, then reconnect power and test.
  • 7
    If the fault persists after the above checks, 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 8 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.