AU
U100308

This DTC indicates abnormal communication between the EPB (Electronic Parking Brake) control module and the vehicle chassis CAN network (Chassis CAN) — Atto 8

Braking System

This DTC indicates abnormal communication between the EPB (Electronic Parking Brake) control module and the vehicle chassis CAN network (Chassis CAN).

Technical details: 1) The EPB module fails to receive valid data frames from the ABS/ESP, VCU, or gateway module within the preset monitoring period (typically 100ms), triggering a timeout counter overflow; 2) Received CAN messages fail the CRC check, contain an abnormal Data Length Code (DLC), or trigger a Bus-Off state.

On the BYD E5, the EPB system relies on the CAN bus to receive the vehicle speed signal (from the ABS), brake pedal status, gear position information, and power mode signal.

Communication interruption causes the EPB auto-release/Auto Vehicle Hold (AVH) function to fail.

Extreme cases may trigger Limp Home mode, though the system usually retains the mechanical emergency release function.

This fault belongs to the chassis network communication category.

Prioritize inspecting the physical layer wiring.

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Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1CAN bus physical layer fault: Includes short circuit between CAN-H and CAN-L, short to 12V power supply/ground, open circuit, or excessive contact resistance (>1Ω). Common causes include wiring wear at the firewall grommet or the floor harness under the door sill trim, and connector water ingress and oxidation after driving through water.
  • 2Abnormal terminal resistor matching: The E5 model chassis CAN bus uses standard 120 Ω terminal resistors located in the ABS control unit and the gateway controller (integrated into the instrument panel distribution box or a standalone module). Total parallel resistance must measure 60 Ω. Resistance deviations exceeding 10% (e.g., poor connections causing resistance >70 Ω or <50 Ω) cause signal reflection and bit errors.
  • 3EPB control module internal fault: Damaged internal CAN transceiver (e.g., NXP TJA1042 chip), crystal oscillator clock drift shifting the sampling point, or failed filter capacitor in the module power supply circuit (12V to 5V) causing excessive power supply ripple.
  • 4Gateway controller routing fault: The E5 uses a gateway to connect the powertrain CAN, chassis CAN, and comfort CAN. A corrupted internal CAN routing table, an outdated software version, or an abnormal gateway power supply interrupts communication between the EPB network segment and other network segments.
  • 5Electromagnetic interference or power supply quality issues: High-frequency interference from the operating high-voltage system (drive motor, DC-DC converter) enters the CAN wiring harness through inductive coupling, or an aging 12V battery (internal resistance >10mΩ) causes the system voltage to drop below 9V during startup, resetting the communication nodes.
What To Do
  • 1
    DTC Confirmation and Freeze Frame Analysis: Use a BYD VDS2000 or Launch X-431 diagnostic tool to read all DTCs. Distinguish between Current and History codes. Record the vehicle speed, system voltage, ambient temperature, and bus load rate from the freeze frame data. Check for accompanying multi-module communication faults (e.g., U1003, U0308, U0101). If present, prioritize troubleshooting the gateway and bus physical layer over individual modules.
  • 2
    EPB module power supply and ground reference check: Disconnect the EPB control module connector (located under the center armrest or near the rear axle motor). Measure the constant power (B+, Pin 30); it must be 12.6V ± 0.3V (static). Measure the ignition power (IGN, Pin 15); it must be >12V with the ignition ON. Measure the resistance between the ground wire (GND) and the vehicle body; it must be <1Ω. Inspect fuses EF05/EF06 (E5 model) in the instrument panel power distribution box for blown elements or poor connections. Verify the power supply waveform ripple is <100mV.
  • 3
    CAN bus physical layer quantitative test: Measure the terminal resistance between OBD diagnostic connector Pin 6 (CAN-H) and Pin 14 (CAN-L). The resistance must be 60Ω ± 6Ω (two 120Ω resistors in parallel). If the resistance is abnormal, disconnect the ABS and gateway modules separately for isolation testing. Measure the CAN-H to ground voltage (2.5-3.5V recessive, 3.5-4.5V dominant) and the CAN-L to ground voltage (1.5-2.5V recessive, 0.5-1.5V dominant). Use an oscilloscope to view the CAN waveform. Check for spikes, bit width distortion, or abnormal voltage amplitude.
  • 4
    Gateway controller in-depth inspection: Access the E5 gateway module (typically integrated into the dashboard power distribution box at position G2D or a standalone gateway box). Verify it has the latest software version (e.g., post-2019 versions resolved early EMC compatibility issues). Check CAN wiring continuity between the gateway, EPB, and ABS (continuity resistance <1Ω, insulation resistance >10MΩ). Use a diagnostic tool to perform a gateway communication test and check the packet loss rate.
  • 5
    Harness interference inspection and rectification: Check if the EPB CAN harness (usually a twisted pair, 20mm twist pitch) runs parallel to the high-voltage harness (orange HV cable) at a distance of <30cm. If present, reroute the harness to maintain >50cm spacing, or install a ferrite core. Check the 12V battery state of health (SOH) using an internal resistance tester. Replace the battery if the CCA value is below 70% of the nominal value or the internal resistance is >10mΩ.
  • 6
    Software update and module replacement: Update the software for the EPB, ABS, and gateway modules (record the original vehicle VIN and configuration code before updating). If the fault persists after the update and ruling out wiring and power supply issues, replace the EPB control module (E5 part number: 5A-3537010). After replacement, perform the EPB calibration procedure: zero-point learning (release to the limit position and record the position sensor value) and clamping force learning (simulate parking and record the motor current curve).
Real-World Cases
BYD DTC AI Analysis

Oxidized gateway controller connector caused intermittent EPB communication fault

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Symptoms: 2018 BYD E5. While driving, the instrument cluster intermittently displayed "Please check EPB system" and the warning light came on intermittently. After parking, the electronic parking brake sometimes failed to release automatically; light throttle input did not trigger auto-release. Diagnosis: Scanned with VDS2000 and retrieved U100308 (current) and U100304 (history). Freeze frame showed 45 km/h and 12.1 V at the time of fault. Initial checks confirmed normal EPB module power supply, but CAN bus measurements showed CAN-H at 2.8 V (high) and CAN-L at 2.2 V (nominal is 2.5 V/2.5 V). Removed the gateway controller on the left side of the dashboard (integrated in the power distribution box) and found green oxidation on pins 4 and 5 (CAN-H/CAN-L) of connector GJ43 from prior water ingress. Solution: Cleaned the pins with electrical contact cleaner (CRC 2-26), applied conductive grease (Stablant 22), reconnected the connector, and torqued the bolt to 5 N·m. Cleared codes and road-tested 20 km with no recurrence. Analysis: The E5 gateway sits low, so the connector oxidizes easily after water ingress or in high humidity, causing CAN signal attenuation and higher bit error rates.
Original source ↗
BYD DTC AI Analysis

Interference between the CAN wiring harness and high-voltage wiring harness caused data frame errors.

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Symptoms: 2019 E5. After DC fast charging, the EPB warning light stayed on. The vehicle remained drivable but the Auto Hold (AVH) function failed. Manual EPB switch operation worked normally. Diagnosis: DTC U100308 stored. Live data showed the EPB module’s Error Counter for vehicle speed signal frames increasing continuously, while the ABS module transmitted normally. Inspection found an aftermarket dashcam with wiring routed from the dashboard to the floor bundled with the high-voltage distribution box HV+ harness; the CAN lines were not twisted pair. Oscilloscope measurement of the CAN bus revealed high-frequency spikes (>1Vpp) synchronized with the DC-DC converter switching frequency (10kHz). Fix: Rerouted the CAN harness and restored the OEM twisted-pair structure (20mm twist pitch). Installed a ferrite core (TDK ZCAT2030-0930) on the EPB module harness end. Flashed updated software to the motor controller (MCU) and EPB module to resolve early-version EMC compatibility issues.
Original source ↗
BYD DTC AI Analysis

EPB control module internal CAN transceiver damaged

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Symptoms: E5 failed to start. The instrument cluster displayed "Brake System Fault", the EPB switch indicator did not light up, and pressing the brake pedal produced no response. Diagnosis: The scan tool could not connect to the EPB module (timeout), though ABS, BMS and VCU communicated normally, indicating the chassis CAN bus had not completely failed. Power supply (12V) and ground (0.2Ω) at the EPB module tested normal. CAN terminal resistance measured 120Ω (normal is 60Ω with parallel termination), indicating the internal CAN transceiver had failed open and could not parallel the termination resistor. The CAN waveform showed the EPB module was not transmitting (no signals in ID range 0x180–0x1FF). Disassembling the EPB module revealed pins 3 (CANH) and 7 (CANL) of the internal TJA1042 chip shorted to ground. Resolution: Replaced the EPB control module assembly (with cables), performed Zero Point Calibration and Clamping Force Learning. After replacement, executed the "EPB Initialisation" procedure using the scan tool; otherwise parking force will be insufficient or abnormal noise will occur.
Original source ↗
BYD DTC AI Analysis

Degraded 12V battery caused unstable communication voltage, triggering U100308.

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Symptoms: 2018 E5. EPB warning light illuminates during cold starts and extinguishes after warming up, accompanied by slow window operation and headlight flicker. Diagnosis: Read DTC U100308 (historic). Checked CAN line resistance and voltage – both within specification. Measured 12V battery static voltage at 11.2V (low; normal should be 12.6V). Voltage dropped to 8.5V during startup (below 9V safe threshold). Used oscilloscope to check EPB module power waveform – found excessive ripple (>500mVpp). Determined EPB module CAN transceiver operates unstably below 9.5V, causing data frame loss and increased error frames, triggering communication timeout faults. Repair: Replaced 12V battery (spec: VRLA 60Ah or AGM 60Ah; recommend OEM Fengfan or Varta). Cleaned negative earth point (left front longitudinal beam) and positive terminal, applied petroleum jelly to prevent oxidation. Cleared DTCs and monitored for 1 month – fault has not reoccurred. Recommend E5 owners check 12V battery health every 2 years to prevent high-voltage system cascade failures caused by low-voltage system faults.
Original source ↗
Other Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.