AU
U043208

DTC U043208 indicates a private CAN network communication fault in the IPB (Intelligent Power Brake) system — Seal U

Braking System

DTC U043208 indicates a private CAN network communication fault in the IPB (Intelligent Power Brake) system.

Specifically, the IPB control unit detects an integrity check failure when receiving data frames from the MMx_TX module (Inertial Measurement Unit, IMU).

This fault involves data link layer errors, including received byte length mismatching the DBC definition (expected 8 bytes, actual length abnormal), Cyclic Redundancy Check (CRC) errors, Alive Counter discontinuity or abnormal jumps, and signal values exceeding the physically valid range (e.g., lateral acceleration >4g or yaw rate >300°/s).

The IMU provides vehicle lateral acceleration, longitudinal acceleration, and yaw rate signals, serving as the core sensor for ESC (Electronic Stability Control), ABS (Anti-lock Braking System), and AEB (Automatic Emergency Braking).

This fault prevents the IPB from obtaining accurate vehicle body attitude data and triggers the brake system degraded protection mode.

Symptoms include restricted ESC function, altered ABS intervention logic, or disabled Automatic Emergency Braking.

The system usually retains basic hydraulic braking functions.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Internal chip fault or firmware corruption in the Inertial Measurement Unit (IMU) causing abnormal output data frame format or CRC calculation error.
  • 2Physical layer fault in the private CAN bus between the IPB and IMU, including poor connections, oxidized pins, damaged shielding causing electromagnetic interference (EMI), or terminating resistor (typically 120Ω) deviation causing signal reflection.
  • 3An outdated Intelligent Power Brake (IPB) control unit software version or a damaged hardware CAN transceiver fails to correctly parse the new IMU data protocol.
  • 4Abnormal IMU sensor power supply: supply voltage below 9V or above 16V, or excessive ground circuit resistance (>100mΩ), disrupting sensor operating timing.
  • 5After vehicle wading or a collision, the IMU mounting base loosens or water enters the sensor, causing the inertial sensing element (MEMS) to output abnormal electrical signals.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS2000/VDS2100) to read the complete fault code tree. Check for accompanying faults such as U043204 (communication timeout) or C0035 (lateral acceleration sensor fault). Record the vehicle speed, yaw rate, and IMU raw signal status at the time of the fault from the freeze frame data.
  • 2
    Check IMU installation status: Confirm IMU retaining bolt torque (usually 8-12 N·m), check the mounting surface for deformation, and verify sensor level error is <2°. Mounting angle deviation causes the system to misjudge the gravity component as lateral acceleration.
  • 3
    Measure the IMU power supply and ground: Disconnect the IMU connector, turn the ignition switch to ON, and measure the voltage between the power pin (usually Pin 1/16) and ground. Standard: 12V ± 0.5V. Measure the ground resistance. Standard: <1Ω. Check the IPB main relay power supply stability.
  • 4
    Check the private CAN bus: Use an oscilloscope to measure the private CAN-H (orange/black) and CAN-L (orange/brown) waveforms between the IPB and IMU. The dominant level should be 2.5-3.5 V (CAN-H) and 1.5-2.5 V (CAN-L), with a recessive level of 2.5 V. Measure the termination resistance; it should be approximately 60 Ω (two 120 Ω resistors in parallel). Check the wiring insulation; resistance to ground should be >1 MΩ.
  • 5
    Perform IMU signal verification: With the vehicle stationary on a level road surface, use the diagnostic tool to read the IMU data stream. Lateral acceleration should be ≈0 m/s² (±0.2 m/s² tolerance), longitudinal acceleration should be ≈0 m/s², and yaw rate should be ≈0°/s. Slowly turn the steering wheel and observe the yaw rate signal to confirm it changes smoothly without jumps or spikes.
  • 6
    Software update and calibration: If the wiring is normal, update the IPB control unit software to the latest version (verify the software number for the vehicle model); perform IMU zero-point calibration (Sensor Neutral Position Calibration) and yaw rate sensor calibration (Yaw Rate Sensor Calibration).
  • 7
    Component replacement and verification: If the fault persists, first replace the IMU sensor (Note: On some models, the IMU is integrated into the IPB; replace the IPB assembly). After replacement, bleed the brake system again and perform an ESC dynamic function test (verify straight-line, turning, and emergency braking conditions).
Real-World Cases
BYD DTC AI Analysis

Song Plus DM-i IMU internal chip intermittently resets causing abnormal data frame length

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Vehicle: 2021 Song PLUS DM-i 110KM Flagship PLUS, 32,000 km. Symptoms: ESC and ABS warning lights illuminate intermittently. Lights clear after ignition restart. Frequency increases at high speed. Diagnosis: DTC U043208 (current). Freeze frame shows 118 km/h at time of fault; IMU status word 0x04 (data length error). IMU supply voltage 13.8 V (normal). Private CAN waveform analysis revealed abnormal 9-byte frames every 5–8 minutes (standard: 8 bytes). Disassembled IMU and found cold solder joint on the decoupling capacitor for the internal MEMS chip supply; chip was resetting sporadically and transmitting error frames. Resolution: Replaced IMU sensor (Part No.: HA2E-3636400), upgraded IPB software to 2023Q2, and performed yaw rate sensor calibration. Road tested 100 km; no recurrence.
BYD DTC AI Analysis

Seal EV: Abnormal private CAN bus termination resistance after accident repair

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Vehicle: 2022 Seal 700km Long Range RWD (rear-wheel drive), accident-repaired. Symptoms: After accident repairs, instrument cluster displayed "Brake System Fault" and automatic emergency braking was unavailable. Retrieved DTCs U043208 (historic fault) and U043204 (current fault). Diagnosis: Front compartment harness inspection found the IPB harness crushed in the accident. Water entered the 120Ω termination resistor connector at the private CAN bus branch, oxidizing the contact and drifting resistance to 85Ω; this caused signal reflection and bit errors. IMU tested normal, but IPB continuously received data frames with CRC errors. Solution: Repaired harness and replaced termination resistor (soldered to IMU harness end). Cleaned IPB and IMU connectors with alcohol and applied conductive grease. Bus resistance restored to 60Ω. Cleared DTCs and passed dynamic testing.
BYD DTC AI Analysis

Qin PLUS DM-i software version mismatch caused counter synchronisation failure

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Vehicle: 2023 Qin PLUS DM-i Champion Edition. Issue appeared after infotainment system upgrade. Symptoms: Day after OTA update to DiLink system, instrument cluster showed "Check Braking System" warning. Current DTC U043208. Diagnosis: Wiring check showed no damage. IMU power supply: 12.2V (normal). Technical bulletin confirms this batch has Alive Counter sync conflict between IPB software V1.02 and new IMU firmware. After 50 consecutive sleep/wake cycles, IPB counter validation logic fails against IMU. Solution: Replaced no hardware. Rolled back IPB software to V1.01, then upgraded to V1.03 patch. Performed 20 sleep/wake cycles; fault code did not return.
BYD DTC AI Analysis

Han EV Genesis Edition: Deformed IMU mounting base caused out-of-range signal values

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Vehicle: 2022 Han EV Genesis Edition 715KM front-wheel-drive flagship, repaired after underbody damage. Symptoms: Normal straight-line driving, but during fast cornering (>60 km/h) the ESC intervened abnormally. The instrument cluster showed a brake system fault; DTC U043208 (signal value error) stored. Diagnosis: The previous underbody impact had slightly deformed the subframe, twisting the IMU mounting bracket (located above the subframe) 2.3°. IMU data read normally at standstill, but cornering coupled gravitational acceleration components and pushed sensor output past the IPB’s 4.5g physical limit, triggering the signal value error. Resolution: Corrected the subframe mounting position, replaced the deformed IMU bracket, and recalibrated the IMU zero point. Road testing included a moose test; ESC operated normally and the fault cleared.
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.