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U055A00

U055A00 is a CAN bus communication protocol layer fault code — Qin Plus

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 Qin Plus indicates a medium-severity CAN communication problem between the VCU and IPB braking system, where a received message has the wrong data length or format. Common causes include a VCU software defect, incorrect software configuration, or outdated IPB software that cannot read the message correctly. Visit an authorised dealer for diagnosis and software update or reprogramming. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1VCU software defect or version mismatch: The VCU control program contains a bug causing the transmitted CAN frame data length field to mismatch the actual payload, or incorrect vehicle model configuration data flashed to the VCU.
  • 2Outdated IPB software version: 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 wiring connections, or abnormal terminating resistance corrupts the data frame length field during transmission.
  • 4Gateway module routing fault: When the VCU and IPB communicate across network segments (via the gateway), the gateway truncates data, generates padding errors, or causes a buffer overflow during 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 when the fault occurred, 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 version numbers. Log in to the BYD TPI (Technical Product Information) system and check for a software upgrade bulletin for this fault code. Confirm software compatibility between both 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 of the VCU (located on the left side of the front compartment) and the IPB (located on the engine compartment front bulkhead). 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 and monitor specific ID messages sent from the VCU to the IPB in real time (such as the 0x130 vehicle status frame and 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 no wiring faults exist, perform a network sleep/wake-up test: disconnect the battery negative terminal for more than 5 minutes, clear all adaptive values and fault codes, restore power, perform the IPB bleeding calibration and steering angle sensor calibration, and conduct 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 to resolve the fault, replace the VCU assembly and perform immobilizer matching and system configuration programming.
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 Qin Plus 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.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.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.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.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.