AU
C05B001

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

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 — Seal U
  • 1Brake caliper (wheel cylinder) piston seal aged, damaged, or improperly installed, causing hydraulic fluid leakage.
  • 2Visible leakage caused by loose connections, corrosion, or physical damage to steel brake pipes or rubber hoses.
  • 3Worn master cylinder seal inside the IPB integrated brake module, or failed valve body seal (internal leakage)
  • 4Overdue brake fluid replacement causes excessive moisture content, resulting in seal swelling, seal corrosion, or metal component rust.
  • 5Excessive brake pad wear causes excessive caliper piston travel, displacing the seal ring and causing 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 to confirm the specific operating conditions of the L1 circuit pressure build-up failure.
  • 3
    Visually inspect the hoses, steel pipes, fittings, and brake wheel cylinder 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 if the pressure drop rate exceeds the limit to locate the leak point.
  • 5
    Disassemble the brake caliper and inspect the piston seal condition, check the sliding pins for binding, and evaluate brake pad wear.
  • 6
    Replace the failed component based on the inspection results: for external line or wheel cylinder leaks, replace the corresponding component; for internal IPB leaks, replace the IPB electro-hydraulic module assembly with ECU.
  • 7
    Bleed the brake system thoroughly according to the BYD standard procedure. Verify no air remains in the L1 circuit and related circuits.
  • 8
    Fill with DOT4 or 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 dynamic road test verification (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 Seal U 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.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.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.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.
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.