AU
C05B001

DTC C05B001 indicates the IPB (Intelligent Integrated Brake System) detects a leak in brake hydraulic circuit L1 — Atto 8

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.

The C05B001 fault code on the Atto 8 indicates a medium-severity leak in brake hydraulic circuit L1 detected by the Intelligent Integrated Brake System. This can increase brake pedal travel, reduce braking force, and affect ABS, ESC, or emergency braking functions. Common causes include a damaged brake caliper piston seal or leaking brake pipe/hose fittings. Visit an authorised dealer for inspection and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 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 lines or rubber hose fittings.
  • 3Worn master cylinder sealing ring or failed valve body seal inside the IPB integrated brake module (internal leakage)
  • 4Failure to replace brake fluid for an extended period causes excessive water content, resulting in seal swelling, corrosion, or metal component rust.
  • 5Excessive brake pad wear causes excessive caliper piston travel, displacing the seal ring and resulting in leakage.
What To Do
  • 1
    Park the vehicle safely, disconnect the high-voltage system power supply, and check if the brake fluid reservoir level has dropped abnormally.
  • 2
    Use the VDS2000/3000 diagnostic tool to read the fault code and freeze frame data, and confirm the specific operating conditions of the L1 circuit pressure build-up failure.
  • 3
    Visually inspect the exterior of the hoses, steel pipes, fittings, and brake wheel cylinders corresponding to the 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 if the pressure drop rate exceeds the limit, and locate the leak.
  • 5
    Disassemble the brake caliper and inspect the piston seal condition, check for sliding pin binding, and check brake pad wear.
  • 6
    Replace the failed component based on the inspection results: if the external lines or wheel cylinder leak, 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 the BYD standard procedure. Ensure no residual air remains in the L1 circuit and related circuits.
  • 8
    Add 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 the fault is resolved.
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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.