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) — Seal U

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 — Seal U
  • 1VCU power supply system fault: Includes a blown VCU constant power fuse (typically EF17/EF18 or a high-current fuse above 30A in the front compartment fuse box), burnt relay contacts, poor power circuit connections, or a loose ground wire. These faults cause the VCU to restart intermittently or freeze, preventing continuous CAN message transmission.
  • 2Power CAN bus physical layer fault: open or short circuit in the CAN_H and CAN_L lines between the VCU and IPB (shorted together or shorted to power/ground), excessive contact resistance (backed-out pins or water ingress causing oxidation), or abnormal terminating resistance (120Ω in parallel should be 60Ω) causing signal reflection.
  • 3VCU software fault or hardware damage: Damaged internal VCU CAN transceiver chip, MCU crash (program runaway), abnormal Watchdog reset, or a software version bug causing an abnormal message transmission cycle.
  • 4IPB reception fault: A damaged internal IPB CAN receiving module, abnormal software parsing, or an abnormal IPB power supply/ground prevents the IPB from correctly receiving VCU messages (although the IPB logs the fault code, the root cause may lie within the IPB itself).
  • 5Network interference or bus congestion: Other malfunctioning nodes on the power CAN bus (e.g., BMS, MCU, 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. Also check for other 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 fuse (should be <0.1V). Check the tightening torque of the VCU ground bolt, usually located on the front left side member or firewall (standard: 10-12 N·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 12V battery negative terminal. 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. 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 should 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 wiring harness connector (located near the brake master cylinder, prone to moisture) and the VCU wiring harness connector (located in the front compartment or behind the glove box) for backed-out pins, enlarged terminals, water corrosion (green rust), and harness damage. Check the front compartment wiring harness for chafing against metal body edges, especially at the firewall pass-through.
  • 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. 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 wiring is normal, update the VCU software to the latest version (e.g., V2.x versions released after 2023 resolve the early communication timeout bug). Verify the VCU and IPB CAN baud rate (typically 500kbps) and network address configurations are correct.
  • 7
    Component replacement and matching: If the above steps fail, first replace the VCU assembly (requires online anti-theft matching and parameter configuration). If the fault persists, replace the IPB assembly (requires brake line bleeding and system calibration). After replacement, perform a network communication test to verify the fault code does not reappear.
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 Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.