AU
U025900

U025900 indicates the IPB (Intelligent Integrated Braking System/Intelligent Power Braking System) failed to receive a valid message or heartbeat signal from the VCU (Vehicle Control Unit) via the powertrain CAN bus within the calibrated time window (typically 100-200ms) — Atto 8

Braking System

U025900 indicates the IPB (Intelligent Integrated Braking System/Intelligent Power Braking System) failed to receive a valid message or heartbeat signal from the VCU (Vehicle Control Unit) via the powertrain CAN bus within the calibrated time window (typically 100-200ms).

In the BYD DM-i hybrid architecture, the VCU acts as the primary vehicle controller.

It calculates the regenerative braking torque request in real time and sends it to the IPB.

The IPB then coordinates the distribution ratio between electric motor braking and hydraulic braking based on this request.

Upon a communication timeout, the IPB determines it has lost communication with the VCU and enters fail-safe/limp-home mode.

The IPB forcibly cancels regenerative braking and retains only basic hydraulic brake assist (vacuum or electric assist, depending on the IPB type) while triggering an ABS/ESC system fault warning.

This U-category communication fault indicates a network or physical layer connection anomaly rather than a functional failure of the braking actuators.

However, it severely impacts regenerative braking efficiency and braking coordination.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1VCU power supply system fault: Causes include a blown VCU constant power fuse (usually EF17/EF18 or a high-current fuse above 30A in the front compartment fuse box), burnt relay contacts, poor power circuit connection, or loose ground wire. These faults cause intermittent VCU restarts or freezing, preventing continuous CAN message transmission.
  • 2Powertrain CAN bus physical layer fault: Open or short circuit (shorted together, to power, or to ground) in the CAN_H and CAN_L lines between the VCU and IPB; excessive contact resistance (backed-out pins, water ingress oxidation); or abnormal terminating resistance (120Ω in parallel should measure 60Ω) causing signal reflection.
  • 3VCU software fault or hardware damage: damaged VCU internal CAN transceiver chip, MCU crash (program runaway), abnormal Watchdog reset, or software version bug causing an abnormal message transmission cycle.
  • 4IPB reception fault: A damaged IPB internal CAN receiving module, abnormal software parsing, or an IPB power supply/ground fault prevents the IPB from correctly receiving VCU messages (although the IPB reports the fault code, the root cause may be within the IPB itself).
  • 5Network interference or bus congestion: Malfunctioning nodes on the powertrain CAN bus (such as the BMS, MCU, or OBC) send error frames, causing excessive bus load and delaying or discarding VCU messages.
What To Do
  • 1
    Fault Confirmation and Freeze Frame Analysis: Use the VDS2000/VDS3000 diagnostic tool to read the complete fault codes and freeze frame data. Record the vehicle speed, SOC, gear position, and brake pedal status at the time of the fault. Determine if the fault is intermittent or current. Check for accompanying U01xx series communication fault codes.
  • 2
    VCU power supply and ground check: Check the VCU constant power fuse (e.g., EF15/EF16 30A) and IGN power supply fuse in the front compartment power distribution box. Measure the voltage drop across the fuses (should be <0.1V). Check the tightening torque of the VCU ground bolt (usually located on the front left longitudinal beam or firewall; standard: 10-12N·m). Measure the supply voltage at the VCU connector pins (should be 9-16V, fluctuation <0.5V).
  • 3
    CAN bus physical layer inspection: Disconnect the battery negative terminal and measure the resistance between pin 6 (CAN_H) and pin 14 (CAN_L) of the OBD connector. The standard value is approximately 60 Ω (two 120 Ω terminating resistors in parallel). Power on the vehicle and measure the CAN_H to ground voltage (2.5-3.5 V) and the CAN_L to ground voltage (1.5-2.5 V). The differential voltage must be between 0.2 V and 2.5 V. Use an oscilloscope to observe the CAN waveform and check for distortion or error frames.
  • 4
    Wiring harness and connector inspection: Focus inspection on the IPB harness connector (located near the brake master cylinder, prone to moisture) and the VCU harness connector (located in the front compartment or behind the glove box) for backed-out pins, spread terminals, water corrosion (green rust), and harness damage. Inspect the front compartment harness for chafing against metal body edges, especially at the firewall opening.
  • 5
    Network topology isolation test: Disconnect other powertrain CAN nodes (BMS, MCU, OBC, etc.) one by one, except the VCU and IPB. Observe if the fault disappears to rule out interference from other modules. Use CANoe or a BYD dedicated network analyzer to capture bus messages and confirm if the VCU periodically sends brake request messages (ID is usually 0x1xx or 0x2xx series, depending on the platform).
  • 6
    Software update and configuration: If the wiring is normal, update the VCU software to the latest version (e.g., V2.x versions released after 2023 fix the early communication timeout bug). Verify the VCU and IPB CAN baud rate (typically 500 kbps) and network address configurations are correct.
  • 7
    Component Replacement and Matching: If the above steps fail, first replace the VCU assembly (perform online anti-theft matching and parameter configuration). If the fault persists, replace the IPB assembly (bleed the brake lines and calibrate the system). After replacement, perform a network communication test to verify the fault code does not return.
Real-World Cases
BYD DTC AI Analysis

Intermittent brake warning light during high-speed driving in Song PLUS DM-i

✓
At highway speeds (80-120km/h), the ABS/ESC warning light intermittently illuminated with loss of regenerative braking. Retrieved DTCs U025900 (VCU communication timeout) and U014600 (lost communication with VCU). Found the VCU constant power fuse (EF17 30A) slightly loose in the fuse box. Vehicle vibration intermittently increased contact resistance (0.5-2Ω), causing momentary VCU power loss and restart. Retightened the fuse base spring clips and replaced the fuse. Fault resolved.
BYD DTC AI Analysis

IPB communication timeout after accident repair

✓
Front-end collision. After repair at a 4S dealership, the U025900 DTC appeared after approximately 50 km. Inspection found the IPB harness connector (at the firewall) was disconnected during the accident repair. Reconnection lacked the locking 'click'; the connector sat in place but the lock did not fully engage, causing intermittent open circuits on rough roads. Reconnected the IPB connector, confirmed the secondary lock engaged, and secured it with waterproof tape. Fault resolved.
BYD DTC AI Analysis

Software version mismatch caused multiple communication failures

✓
A batch of Song PLUS DM-i vehicles developed multiple U025900 faults after an OTA update. Investigation found the VCU upgrade changed the message transmission cycle from 10ms to 20ms, while early IPB versions used a 15ms reception timeout threshold, causing false timeouts. Resolve by rolling back the VCU software or flashing the IPB firmware to adjust the timeout threshold to 50ms. This is a software calibration compatibility issue.
BYD DTC AI Analysis

VCU main board CAN transceiver chip failure

✓
Vehicle failed to enter Ready mode; the instrument cluster displayed "Brake System Fault". The scanner retrieved DTC U025900, which would not clear. VCU power supply tested normal, but powertrain CAN waveform analysis showed no signal output from the VCU transmitter (other nodes were normal). VCU disassembly revealed pins 3 and 7 (CAN_H/CAN_L) of the TJA1043 CAN transceiver on the main board were shorted to ground. Replacing the VCU main board (or assembly) resolved the fault. Root cause: ESD damage to the chip.
BYD DTC AI Analysis

IPB internal communication module fault

✓
After sitting overnight, the vehicle set DTC U025900 immediately on startup and the code would not clear. The VCU CAN output tested normal (oscilloscope showed normal waveform at the VCU connector), but no waveform reached the IPB CAN input, indicating an open in the intermediate harness. Further inspection found micro-cracks in the copper traces of the CAN signal input circuit on the IPB internal PCB (likely from brake master cylinder vibration), interrupting the signal. Replaced the IPB assembly and performed the bleed calibration to resolve.
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.