AU
C05B001

DTC C05B001 indicates the IPB (Intelligent Integrated Brake System) detects a leak in brake hydraulic circuit L1 — Atto 3

Braking System

DTC C05B001 indicates the IPB (Intelligent Integrated Brake System) detects a leak in brake hydraulic circuit L1.

The IPB is the BYD brake-by-wire system (One-Box architecture) integrating brake assist, Electronic Stability Control (ESC), and energy recovery functions.

Circuit L1 typically refers to the primary brake circuit or a specific wheel cylinder control circuit (e.g., front left wheel circuit).

This fault indicates hydraulic system sealing failure.

This condition can extend brake pedal travel, cause a non-linear reduction in braking force output, and restrict or disable active safety functions including ABS, ESC, and Automatic Emergency Braking.

Due to braking safety risks, the system illuminates multiple warning lights and may limit vehicle power output.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Brake caliper (wheel cylinder) piston seal aged, damaged, or improperly installed, causing hydraulic fluid leakage.
  • 2Visible leak caused by loose, corroded, or physically damaged steel brake pipes, rubber hoses, or fittings.
  • 3Worn master cylinder seal or failed valve body seal inside the IPB integrated brake module (internal leakage)
  • 4Prolonged failure to replace brake fluid causes excessive moisture content, leading to seal swelling, corrosion, or rusted metal components.
  • 5Excessive brake pad wear extends brake caliper piston travel, displacing the seal and causing a leak.
What To Do
  • 1
    Park the vehicle safely, disconnect the high-voltage power supply, and check if the brake fluid level in the reservoir has dropped abnormally.
  • 2
    Use the VDS2000/3000 diagnostic tool to read the fault code and freeze frame data to confirm the specific operating conditions when the L1 circuit failed to build pressure.
  • 3
    Visually inspect the hoses, steel pipes, fittings, and brake wheel cylinders of the corresponding L1 circuit lines (usually involving the front left wheel or main circuit) for signs of fluid leakage.
  • 4
    Perform the brake system pressure hold test. Use the diagnostic tool to drive the IPB motor to build hydraulic pressure. Observe whether the pressure drop rate exceeds the limit to locate the leak point.
  • 5
    Disassemble and inspect the brake caliper piston seal condition, caliper sliding pin binding, and brake pad wear.
  • 6
    Replace the failed component based on the inspection results: if an external line or wheel cylinder leaks, replace the corresponding component; if the IPB leaks internally, replace the IPB electro-hydraulic module assembly with ECU.
  • 7
    Thoroughly bleed the brake system according to BYD standard procedures. Verify the L1 circuit and related circuits are free of residual air.
  • 8
    Fill with DOT4 or the specified brake fluid to the MAX level, then perform the IPB system self-learning and pressure calibration procedure.
  • 9
    Clear the fault code, perform a static function test (pedal feel check) and a dynamic road test (ABS activation test), and confirm fault resolution.
Real-World Cases
BYD DTC AI Analysis

BYD Seal 06 DM-i Brake Hydraulic Circuit L1 Leak Repair

✓
The dashboard showed a constant brake system warning light and brake pedal travel increased noticeably. The scan tool retrieved DTC C05B001 (brake hydraulic circuit L1 leak), along with abnormally low brake fluid level. During diagnosis, the technician confirmed the DTC and freeze frame data. Visual inspection revealed brake fluid leak traces at the left front brake caliper. Disassembly revealed the caliper piston seal cracked from aging. The technician replaced the left front brake caliper assembly and damaged seal, thoroughly bled the brake system, and topped up the brake fluid to the standard level. After the repair, the technician cleared the DTC. A road test confirmed the hydraulic circuit sealed normally and ABS functioned properly.
Original source ↗
BYD DTC AI Analysis

Atto 3 (Yuan Plus) brake hydraulic circuit L1 leak diagnosis

✓
The instrument cluster displayed a brake system fault. The scanner showed C05B001 (Brake Hydraulic Circuit L1 Leaky) with occasional abnormal ESP intervention. During diagnosis, a full code scan revealed C2A1700 (Brake Hydraulic Circuit L1 Overcompensated/Air Present). The brake booster motor and pressure sensor checked normal. Pressure-testing brake hydraulic circuit L1 showed the pressure wouldn’t hold. Further inspection found leakage at the right rear wheel cylinder hydraulic line connection and a worn piston seal. Tightened and replaced the leaking hydraulic line fitting, replaced the right rear brake wheel cylinder assembly, and bled the brake system to clear all air.
Original source ↗
BYD DTC AI Analysis

Comprehensive Multi-circuit Leak Diagnosis and System Maintenance

✓
The vehicle had simultaneous DTCs C05B001 (L1 circuit leak) and C05B000 (L2 circuit leak), a spongy brake pedal, uneven brake force distribution, and multiple brake warning lights illuminated on the dashboard. During diagnosis, the technician thoroughly inspected the brake hydraulic circuits and found physical damage and corrosion on several lines, plus contaminated fluid in the brake fluid reservoir. The technician ran a brake system self-test with the diagnostic tool and observed slow pressure build-up and an inability to hold pressure. Inspection revealed severely contaminated brake fluid (moisture content exceeded specifications), causing multiple seals to swell and leak. The repair involved replacing all aged brake hoses and seals, thoroughly flushing and replacing the brake fluid throughout the vehicle, and replacing the brake fluid reservoir filter. The ABS pump also showed minor internal leak signs, so the technician recommended replacing the ABS/ESP assembly. The system returned to normal after the repair.
Original source ↗
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.