AU
C05B000

DTC C05B000 indicates the IPB (Intelligent Integrated Braking System) detects abnormal pressure or a leak in the L2 hydraulic circuit (typically the right-rear/left-rear brake circuit, depending on vehicle brake line routing) — Seal U

Braking System

DTC C05B000 indicates the IPB (Intelligent Integrated Braking System) detects abnormal pressure or a leak in the L2 hydraulic circuit (typically the right-rear/left-rear brake circuit, depending on vehicle brake line routing).

The internal IPB hydraulic pressure sensor triggers this fault upon detecting an inability to maintain L2 circuit pressure or an abnormal pressure drop during pressure build-up.

The L2 circuit is the second hydraulic circuit in the dual-circuit braking system and controls brake pressure for specific wheels.

If the IPB detects a circuit pressure decay rate exceeding the threshold (typically >3-5 bar/s) or a failure to reach target pressure, it records the fault and enters fail-safe mode.

This mode limits or disables functions including ABS, ESC, and Automatic Emergency Braking, while retaining basic hydraulic braking capability.

This hard fault affects driving safety; clearing the DTC will not resolve the issue.

The C05B000 fault code on the Seal U indicates abnormal pressure or a leak in the L2 brake hydraulic circuit, triggering IPB braking system fail-safe mode. Severity is medium because ABS, ESC, or emergency braking functions may be limited, though basic braking remains. Common causes include internal IPB seal failure or damaged brake lines/hoses causing fluid loss. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Aged or damaged internal valve body sealing ring in the IPB electro-hydraulic module causes internal pressure loss (common in vehicles used for over 2-3 years, especially those operating frequently in high-temperature and high-humidity environments).
  • 2Physical damage to L2 circuit external lines or hoses (including metal line corrosion, rubber hose cracking, or loose fittings), causing brake fluid leakage.
  • 3Brake fluid contamination or degradation causes the IPB internal solenoid valve to stick and fail to seal properly, resulting in pressure crossover between circuits or internal leakage.
  • 4Damaged wheel cylinder (brake caliper) piston seal, causing brake fluid to leak onto the brake disc or externally.
  • 5IPB pressure sensor internal fault or abnormal signal triggering a false leak warning (verify by comparing with a pressure tester).
What To Do
  • 1
    Connect the VDS or a dedicated diagnostic tool to read all fault codes. Check for C05B000 and related fault codes (such as C05B001, C055E00, etc.), and record the pressure value and vehicle speed from the freeze frame data.
  • 2
    Visually check the brake fluid reservoir level. If the level is significantly below the MIN mark, inspect the L2 circuit (right rear/left rear) lines, hoses, calipers, and the IPB unit for wet spots or fluid drips.
  • 3
    Raise the vehicle and remove the left and right rear wheels. Inspect the brake hoses, hard line fittings, and wheel cylinder dust boots for signs of brake fluid leakage. Use a UV leak detection lamp if necessary.
  • 4
    If visual inspection shows no leaks, connect the brake system pressure tester and perform the IPB hydraulic unit active test. Perform a pressure holding test on the L1 and L circuits separately (target pressure 100-150 bar, hold for 10 seconds) and observe if the pressure drop exceeds the standard value (typically <5 bar is normal).
  • 5
    If diagnostics confirm an internal IPB leak, replace the IPB electro-hydraulic module assembly with ECU (part numbers vary by vehicle model; for example, Song PLUS DM-i uses the 6A-3555010 series). After replacement, perform the IPB bleeding procedure, longitudinal sensor calibration, yaw rate sensor calibration, and pedal position learning.
  • 6
    If the external lines or wheel cylinder leak, replace the damaged flexible hose, rigid pipe, or wheel cylinder repair kit. After replacement, tighten to the standard torque (hose fittings usually 15-18 N·m, wheel cylinder bolts usually 30-35 N·m) and perform the standard brake system bleeding procedure.
  • 7
    After repair, clear the fault code and perform a road test (including an ABS activation test at speeds above 30 km/h). Confirm the fault code does not return and the instrument cluster warning light is off.
Real-World Cases
BYD DTC AI Analysis

Song Plus DM-i: IPB internal valve body leak causing C05B000

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Vehicle: 2021 Song PLUS DM-i, 32,000 km. Symptoms: ABS, ESP and Automatic Emergency Braking warning lights illuminated simultaneously. VDS read DTC C05B000 (hydraulic circuit L2 leak). Diagnosis: Brake fluid level normal. Visual inspection of L2 circuit (right rear/left rear) showed no leakage. Pressure hold test on IPB using pressure tester: L2 circuit pressure dropped from 150 bar to 80 bar in 5 seconds; L1 circuit held pressure normally. Disassembled IPB and found internal valve body seal ring aged and deformed. Solution: Replaced IPB assembly. Performed VDS online configuration (wrote IPB configuration code, calibrated sensors, ran bleed procedure). Fault cleared.
BYD DTC AI Analysis

Burst right rear brake hose caused hydraulic circuit fault

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Vehicle: 2022 BYD Song Pro DM-i, 48,000 km, coastal city use. Symptoms: While driving, the dash suddenly showed a brake system fault, then the ABS warning light lit up and brake pedal travel increased. Diagnosis: Scanned DTC C05B000. Found brake fluid traces on the right rear brake hose. Lifted the vehicle and found cracks near the body mounting point leaking brake fluid—salt spray corrosion had aged the rubber. Repair: Replaced the right rear brake hose (part number SA-3506800), topped up DOT4 brake fluid, performed a conventional bleed, cleared the fault codes and road tested—normal.
BYD DTC AI Analysis

Brake fluid contamination caused the IPB valve body to stick, triggering false warnings.

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Vehicle: 2020 Han EV, 56,000 km, non-authorised service history. Symptoms: Brake system warning light intermittent; obvious at cold start, sometimes clears after warming up. Diagnosis: Read code C05B000. Visual inspection found no external leaks. Pressure testing revealed abnormal fluctuations in the L2 circuit. Removed the IPB inlet filter and found heavy gum deposits; brake fluid was dark brown (severely degraded). Cause: substandard brake fluid and missed service intervals, causing valve sticking. Repair: Thoroughly flushed brake lines, replaced IPB assembly and brake fluid, performed full bleed and calibration. Advised customer to change brake fluid every 2 years.
BYD DTC AI Analysis

Replaced brake pads without bleeding brakes, triggering system warning

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Vehicle: 2021 Tang DM-i, 21,000 km. Symptoms: After replacing rear brake pads at an independent workshop, the dash displayed a brake system fault with DTC C05B000. Diagnosis: Checked brake fluid level – normal, with no leaks evident. Inquiry revealed the technician had disconnected brake hoses without performing the IPB bleed procedure, and the brake pedal felt spongy. Connected a scan tool and executed the IPB bleed procedure, operating the brake pedal as prompted. When the L2 circuit bleed valve opened, no fluid flowed, confirming air had entered the system. Resolution: Used dedicated bleeding equipment to bleed the IPB and all four calipers, performed sensor calibration, and cleared the fault.
BYD DTC AI Analysis

Damaged left rear wheel cylinder seal caused brake circuit pressure loss.

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Vehicle: 2019 Qin Pro EV, 68,000 km. Symptoms: Parking brake intermittently failed to release fully; dashboard showed intermittent warnings before the warning light stayed on constantly. Diagnosis: Retrieved DTCs C05B000 and C12F909 (circuit blockage — actually a secondary fault from insufficient pressure). Inspection found heavy oily sludge on the inner side of the left rear wheel, a torn wheel cylinder dust boot, and brake fluid that had leaked onto the brake disc and inside the wheel hub. Repair: Replaced the left rear brake wheel cylinder assembly, cleaned oil contamination from the brake disc surface (replaced brake pads if necessary), topped up brake fluid, performed the IPB bleed procedure, cleared the fault codes, and tested. Parking brake and service brake functions returned to normal.
Other Seal U Fault Codes
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.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.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.