AU
U055A00

U055A00 is a CAN bus communication protocol layer fault code — Seal U

Braking System

U055A00 is a CAN bus communication protocol layer fault code.

It indicates the IPB (Intelligent Integrated Braking System/One-Box) detects that the data frame length (DLC) of a received VCU (Vehicle Control Unit) CAN message does not match the specified protocol, or the data payload length check fails.

This network communication data link layer error means the VCU transmitted an abnormal message structure (e.g., sending 6 bytes instead of the protocol-specified 8 bytes, or using incorrect padding bytes).

In Song PLUS DMi models, the VCU and IPB communicate via the chassis CAN or powertrain CAN to transmit key signals such as vehicle speed, torque request, and regenerative braking level.

If the data length is incorrect, the IPB cannot correctly interpret the vehicle status.

This can trigger braking system downgrade protection, limit ESC, ABS, and regenerative braking functions, and severely compromise driving safety.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1VCU software defect or version mismatch: The VCU control program contains a bug where the transmitted CAN frame data length field does not match the actual payload, or the VCU contains incorrectly flashed vehicle model configuration data.
  • 2IPB software version too old: Brake system software is incompatible with the new VCU communication protocol, causing standard data frame parsing logic errors.
  • 3CAN network physical layer fault: Electromagnetic interference on the powertrain CAN bus, a poor connection, or an abnormal terminating resistor corrupts the data frame length field during transmission.
  • 4Gateway module routing fault: If the VCU and IPB communicate across network segments (via gateway forwarding), data truncation, padding errors, or buffer overflows occur during gateway data forwarding.
  • 5Abnormal power supply voltage: Unstable VCU or IPB power supply (12V battery voltage below 9V or above 16V) causes the communication controller to transmit and receive abnormal data frames.
What To Do
  • 1
    Use the BYD VDS1000/VDS2000 diagnostic tool to read the complete fault codes and freeze frame data. Record key parameters during the fault, such as vehicle speed, gear position, and CAN communication load. Check for accompanying communication fault codes (e.g., U01A0, U0100).
  • 2
    Check the VCU and IPB software versions. Log in to the BYD TPI (Technical Product Information) system to check for software update bulletins related to this fault code. Confirm software compatibility between the two control units.
  • 3
    Measure the termination resistance between CAN-H and CAN-L at the diagnostic port (OBD) (standard value 60Ω ± 5Ω, single module 120Ω) and the static voltage (CAN-H approx. 2.6V, CAN-L approx. 2.4V, differential voltage approx. 0.2V). Use an oscilloscope to check the CAN waveform for abnormal spikes or bit stuffing errors.
  • 4
    Check the power supply circuits (constant power B+, ignition power IG) and ground points G202 and G301 of the VCU (located on the left side of the front compartment) and the IPB (located on the front bulkhead of the engine compartment). Verify the voltage remains stable within 12V±0.5V to rule out communication faults caused by power supply voltage drop.
  • 5
    Perform network communication monitoring: Select the 'Bus Analysis' function in VDS to monitor specific ID messages sent by the VCU to the IPB in real time (such as 0x130 vehicle status frame and 0x180 torque request frame). Verify the DLC field is the standard length specified by the protocol (usually 8 bytes).
  • 6
    If the software version is normal and no wiring faults exist, perform a network sleep/wake test: disconnect the battery negative terminal for more than 5 minutes, clear all adaptive values and fault codes, and reconnect power. Next, perform the IPB bleeding calibration and steering angle sensor calibration. Perform a road test (including rapid acceleration, energy recovery, and emergency braking conditions).
  • 7
    If the fault recurs intermittently, refer to the workshop manual to first update the VCU software to the latest version (some batches require a simultaneous IPB update). If the update fails, replace the VCU assembly, perform immobilizer matching, and write the system configuration.
Real-World Cases
BYD DTC AI Analysis

After VCU software upgrade on the Song PLUS DM-i, the IPB reported a data length error.

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Vehicle came in for routine maintenance. The technician upgraded the VCU software to optimise the energy recovery strategy but failed to check the IPB version. The next day, the customer reported intermittent ABS and ESC warning lights on the dash while driving. VDS scanning showed IPB code U055A00 (current fault); the VCU showed no faults. Technicians found the VCU now ran V3.2.1 while the IPB remained on old V2.1.0. The new VCU had modified the 0x180 message data structure (adding a 2-byte reserved field), but the IPB detected a length mismatch when parsing with the old protocol. After upgrading the IPB software to V2.5.0, the fault codes cleared and a 20 km road test showed no recurrence.
BYD DTC AI Analysis

Multiple braking system warning lights illuminated after driving on rough roads.

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The customer reported that after driving over a speed bump, the instrument cluster suddenly displayed "Check Brake System" and "Check ESP". VDS detected U055A00 (historical fault) in the IPB with abnormal CAN signals. Found poor contact at the chassis CAN harness connector (GJB01) on the left front longitudinal beam; internal terminals were loose and the fault returned when wiggling the harness. Disassembling the connector revealed that seal deterioration allowed water ingress, oxidizing the terminals. Repaired the terminals, applied conductive grease, replaced the waterproof seal, and wrapped the connection. CAN resistance measured normal and the fault cleared.
BYD DTC AI Analysis

Gateway module failure caused abnormal communication between network segments.

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Song PLUS DMi intermittently showed simultaneous warning lights for the powertrain, brake, and steering systems. VDS scan revealed IPB U055A00, TCU U0100, and communication timeouts in multiple modules. Testing at the diagnostic port revealed an intermittent internal short in the Gateway Module (GWM, located below the dashboard) between Powertrain CAN and Chassis CAN, truncating data frames from the VCU to the IPB during routing. Replaced the gateway controller, configured vehicle parameters and the CAN network matrix online, and restored normal communication across all systems. Fault completely resolved.
BYD DTC AI Analysis

Aftermarket dashcam causing CAN bus interference

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After fitting an aftermarket 360-degree surround view camera system, the ESC warning light occasionally illuminated and cleared after a restart. VDS retrieved IPB history DTC U055A00. Inspection revealed the installer had bundled the dash cam power cable with the CAN bus harness (twisted pair) when routing through the A-pillar, and tapped power from the IPB supply fuse, creating electromagnetic interference. Oscilloscope monitoring revealed periodic spike interference on the CAN bus. Rerouted the wiring loom, kept CAN lines more than 20cm from high-voltage and power cables, and ran a separate power feed for the dash cam. The fault cleared.
BYD DTC AI Analysis

VCU hardware fault generating continuous error frames

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The vehicle failed to power up the high-voltage system. The instrument cluster displayed "Powertrain System Fault" and "Braking System Fault". VDS scanning revealed intermittent loss of communication with the VCU, and the IPB showed current fault code U055A00. Checked VCU power supply and ground — both normal. However, CAN bus monitoring revealed the VCU continuously transmitting malformed data frames (DLC=8 but only 3 bytes of actual data, with CRC errors). Disassembled the VCU and found the internal CAN transceiver chip (TJA1145) damaged, causing the bus fault. Replaced the VCU assembly, performed immobilizer matching, VIN programming and system configuration. Vehicle returned to normal and fault codes have not reappeared.
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.