AU
U100308

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

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.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1CAN bus physical layer fault: Includes short circuit between CAN-H and CAN-L, short to 12V power/ground, open circuit, or excessive contact resistance (>1Ω). Common causes include wire chafing at the firewall wiring grommet or where the floor harness passes under the sill trim, and connector oxidation from water ingress after driving through water.
  • 2Abnormal terminal resistor matching: The E5 model chassis CAN bus uses standard 120Ω terminal resistors located inside the ABS control unit and the gateway controller (integrated into the dashboard power distribution box or as a standalone module). Total parallel resistance must measure 60Ω. Resistance deviations exceeding 10% (e.g., poor connections causing resistance >70Ω or <50Ω) cause signal reflection and bit errors.
  • 3EPB control module internal fault: damaged internal CAN transceiver (e.g., NXP TJA1042 chip), crystal oscillator clock drift causing sampling point offset, or failed module power supply circuit (12V to 5V) filter capacitor causing excessive 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 A corrupted internal CAN routing table, outdated software, or an abnormal gateway power supply interrupts communication between the EPB network segment and other network segments.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 via 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 concurrent 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 box or near the rear axle motor). Measure the constant power supply (B+, Pin 30); voltage must be 12.6V±0.3V (static). Measure the ignition power supply (IGN, Pin 15) with the ignition in the ON position; voltage must be >12V. Measure the resistance between the ground wire (GND) and the vehicle body; resistance must be <1Ω. Inspect fuses EF05/EF06 (E5 model) in the instrument panel distribution box for blown elements or poor connections. Verify the power supply waveform ripple is <100mV.
  • 3
    CAN bus physical layer quantitative inspection: Measure the terminating 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 voltage to ground (2.5–3.5V recessive, 3.5–4.5V dominant) and the CAN-L voltage to ground (1.5–2.5V recessive, 0.5–1.5V dominant). Use an oscilloscope to view the CAN waveform and 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 runs the latest software version (post-2019 versions resolve early EMC compatibility issues). Check CAN line continuity between the gateway, EPB, and ABS (continuity resistance <1Ω, insulation resistance >10MΩ). Perform a gateway communication test using the diagnostic tool and check the packet loss rate.
  • 5
    Harness interference inspection and rectification: Check if the EPB CAN harness (usually a twisted pair, 20 mm twist pitch) runs parallel to the high-voltage harness (orange HV cable) at a distance of <30 cm. If so, reroute the harness to maintain >50 cm 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 falls below 70% of the rated value or if the internal resistance is >10 mΩ.
  • 6
    Software flashing and module replacement: Flash the EPB, ABS, and gateway modules (record the original vehicle VIN and configuration codes before flashing). If the fault persists after flashing 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 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.