AU
U055A00

U055A00 is a CAN bus communication protocol layer fault code — Atto 3

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.

The U055A00 fault code on the Atto 3 indicates a medium-severity CAN bus communication fault where the braking system receives a VCU message with the wrong data length or payload format. Common causes include a VCU software defect, incorrect vehicle configuration, or outdated IPB braking system software. The recommended fix is to visit an authorised dealer for diagnosis and software updates. Typical repair cost data is available for this fault.
5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1VCU software defect or version mismatch: The VCU control program contains a bug, the transmitted CAN frame data length field does not match the actual payload, or incorrect vehicle model configuration data was flashed.
  • 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, poor connections, or abnormal terminal resistance corrupts the data frame length field during transmission.
  • 4Gateway module routing fault: If the VCU and IPB communicate across network segments (via gateway forwarding), the gateway truncates data, causes padding errors, or experiences a buffer overflow during 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 complete fault codes and freeze frame data. Record key parameters at the time of the fault, such as vehicle speed, gear position, and CAN communication bus load. Check for accompanying communication fault codes (e.g., U01A0, U0100).
  • 2
    Check the VCU and IPB software version numbers. Log in to the BYD TPI (Technical Product Information) system to check for software upgrade bulletins for this fault code. Verify software compatibility between the two vehicle 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+ and ignition power IG) and ground points G202 and G301 for 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 drops.
  • 5
    Perform network communication monitoring: Select the 'Bus Analysis' function in VDS to monitor specific ID messages sent from the VCU to the IPB in real time (e.g., 0x130 vehicle status frame, 0x180 torque request frame). Verify the DLC field matches the standard length specified by the protocol (usually 8 bytes).
  • 6
    If the software version is correct and there are no wiring faults, perform a network sleep/wake-up test: disconnect the battery negative terminal for more than 5 minutes, clear all adaptive values and fault codes, and after restoring power, perform the IPB bleed 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, first update the VCU software to the latest version according to the workshop manual (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.

✓
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.

✓
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.

✓
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

✓
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

✓
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 Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.