AU
C104400

DTC C104400 indicates the IPB (Integrated Power Brake) system detects an incomplete or insufficient hydraulic brake line bleeding procedure — Atto 3

Braking System

DTC C104400 indicates the IPB (Integrated Power Brake) system detects an incomplete or insufficient hydraulic brake line bleeding procedure.

The IPB system uses internal pressure sensors to monitor the hydraulic pressure build-up characteristics of each brake wheel cylinder.

When the system detects air in the lines (abnormal compressibility) or identifies an incorrectly executed bleeding procedure, it triggers this fault code and enters fail-safe mode.

This fault restricts ABS/ESP functions, causes abnormal brake pedal travel (spongy pedal), and derates or disables the Automatic Emergency Braking (AEB) function, severely impacting braking safety and driver assistance system performance.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Failure to perform the standard bleeding procedure after brake system repairs (replacing brake fluid, brake hoses, brake calipers, or brake pads), or manual interruption of the bleeding process.
  • 2During the IPB electro-hydraulic module internal bleeding procedure, battery voltage dropped below 12 V or diagnostic communication failed, causing a calibration data write failure.
  • 3Using non-dedicated equipment to bleed the brakes (such as traditional pedal pumping) fails to trigger the IPB active pressure-build bleeding mode, preventing the system from recognizing the bleeding completion status.
  • 4Severe brake fluid loss or a hydraulic line leak prevents the bleeding procedure from building sufficient system pressure to complete the self-check.
  • 5Outdated IPB control unit software version, or abnormal internally stored bleed status flag (false positive fault).
What To Do
  • 1
    Preliminary inspection: Confirm the brake fluid level is between MAX and MIN, check all four wheel brake lines for leaks, measure battery voltage (≥12.5V), and verify normal diagnostic tool communication.
  • 2
    Enter bleeding mode: Use the BYD VDS or Bosch dedicated diagnostic tool to access the IPB system → Special Functions → Hydraulic Brake Bleeding/Refill Procedure. Strictly follow the prompted sequence (usually right rear, left rear, right front, left front).
  • 3
    Perform active bleeding: Connect the diagnostic tool and follow the on-screen prompts to bleed each wheel individually. The IPB motor actively builds high pressure (approximately 20-30 bar). Open the corresponding wheel cylinder bleed screw within the specified time until fluid flows out without bubbles.
  • 4
    Complete calibration: After bleeding all four wheels, use the diagnostic tool to perform 'Bleeding Completion Confirmation' and 'System Pressure Calibration'. Verify successful zero-point calibration of the internal IPB pressure sensor. Read the data stream to confirm the four-wheel pressure build-up time difference is <50ms.
  • 5
    Function verification: Clear the fault code and perform a road test (vehicle speed > 40 km/h). Test ABS activation, verify the instrument cluster ABS/ESC warning light turns off, and confirm the Automatic Emergency Braking (AEB) function is restored.
Real-World Cases
BYD DTC AI Analysis

Electronic bleed procedure not performed after brake fluid replacement on Song PLUS DM-i

✓
After bringing the vehicle in for maintenance and brake fluid replacement, the instrument cluster displayed 'Brake System Fault' with DTC C104400. The technician only performed conventional manual pedal bleeding without executing the IPB active bleeding procedure via VDS. They then used the diagnostic tool to access the IPB system and executed the 'Hydraulic Brake Bleeding' special function, completing active bleeding in the right-rear, left-rear, right-front, left-front sequence. The DTC cleared automatically and brake pedal firmness returned to normal.
BYD DTC AI Analysis

Battery discharged during repair, interrupting the exhaust process

✓
Replaced the left rear wheel cylinder due to a brake fluid leak. Did not connect a stabilized power supply during the repair. During brake bleeding, battery voltage dropped to 10.8V, causing the IPB control unit to restart and set C104400, which could not be cleared. Inspection found the IPB internal bleed counter abnormal. Connected a stabilized power supply to maintain voltage above 13V, performed the complete four-wheel brake bleeding procedure again, and executed 'control unit reset' and 'longitudinal and lateral sensor calibration' after bleeding was complete. Fault resolved.
BYD DTC AI Analysis

False exhaust fault due to IPB software false alarm

✓
No prior service history. ABS warning light intermittent. Read fault code C104400; brake pedal felt normal. No brake fluid leaks. Attempted bleed procedure; display indicated "completed" but fault code persisted. Diagnosis found IPB software was early version with a bug. Solution: Upgraded IPB control unit software to latest version (via BYD OTA or offline programming). Fault code cleared after upgrade. No recurrence after one week of monitoring.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.