AU
C055F00

DTC C055F00 indicates the IPB (Intelligent Integrated Brake System) detects a hydraulic circuit abnormality — Seal U

Braking System

DTC C055F00 indicates the IPB (Intelligent Integrated Brake System) detects a hydraulic circuit abnormality.

The IPB is BYD’s electro-hydraulic integrated braking system (One-Box architecture), integrating brake boost, ESP, ABS, and EPB functions.

This fault occurs when, during a self-check or braking request, the system detects the deviation between the master cylinder pressure sensor feedback and the target pressure exceeds the limit, the hydraulic pump fails to build the required pressure within the specified time, or solenoid valve actuation feedback is abnormal.

Possible causes include hydraulic pump motor failure, pressure sensor drift, a sticking solenoid valve, brake fluid leakage/air lock, or an internal control unit sampling circuit fault.

When this fault occurs, the system may enter Limp Home mode, limiting brake boost or switching to mechanical backup mode, severely affecting braking performance.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Carbon brush wear or rotor seizure in the internal DC motor (hydraulic pump) of the IPB electro-hydraulic module, preventing hydraulic pressure buildup.
  • 2Master cylinder pressure sensor (M/C Pressure Sensor) signal drift or internal open circuit; deviation between the feedback pressure value and actual value exceeds the ±0.5MPa threshold.
  • 3Brake line leak or severely low brake fluid causing system pressure hold test failure (leak rate >0.1MPa/s)
  • 4Open circuit in the solenoid valve coil (such as main valve MV or inlet valve INLET) or stuck valve spool prevents proper regulation of wheel cylinder pressure.
  • 5IPB control unit internal PCB drive circuit fault (e.g., burnt MOSFET) or abnormal ADC sampling circuit.
What To Do
  • 1
    Use the VDS2000/Launch diagnostic tool to read all fault codes and freeze frame data. Record key data from when the fault occurred, including vehicle speed, master cylinder pressure, motor current, and solenoid valve status.
  • 2
    Visually inspect the brake fluid level (must be between MAX and MIN). Inspect the IPB unit, four-wheel brake lines, and hose connections for leaks or signs of fluid seepage.
  • 3
    Run the 'Hydraulic Test' function. Listen for the hydraulic pump operating sound inside the IPB. Observe the pressure build-up time (normally <300ms to reach 10MPa). Check for a timeout or abnormal current (>15A).
  • 4
    Check the IPB supply voltage (12V±0.5V), ground resistance (<1Ω), and CAN-H/CAN-L waveform (2.5V reference, dominant level CAN-H 3.5V/CAN-L 1.5V).
  • 5
    Disconnect the IPB low-voltage wiring harness connector and inspect the pins for oxidation or backing out; measure the master cylinder pressure sensor signal voltage (0.5V-4.5V corresponds to 0-25MPa, approximately 0.8-1.2V when static).
  • 6
    If the above checks are normal, perform the Bleeding Procedure to remove air from the lines, then retest. If the fault persists, replace the IPB integrated electro-hydraulic module assembly.
  • 7
    After replacement, perform "System Calibration" including: pressure sensor zero-point calibration, motor position learning, and solenoid valve self-check. Finally, perform a road test to verify ABS/ESP functions.
Real-World Cases
BYD DTC AI Analysis

Song PLUS DM-i: Brake pedal suddenly went stiff during high-speed driving

✓
At 30,000 km, while driving at highway speed the dashboard suddenly displayed 'Braking System Fault'. The brake pedal became stiff with reduced travel, requiring heavy pedal effort to achieve braking force. Retrieved DTC C055F00. Freeze frame data showed master cylinder pressure at only 0.8 MPa when the fault occurred (target 12 MPa), with motor current at 18 A (over limit). Disassembled the IPB module and found the internal hydraulic pump motor carbon brushes completely worn and the commutator scored. Replaced the IPB assembly and performed the calibration procedure. Pressure build-up time returned to 200 ms. Fault resolved.
BYD DTC AI Analysis

Qin PLUS EV: Hydraulic fault after brake fluid replacement

✓
The vehicle stored DTC C055F00 and the ABS light stayed on after a brake fluid replacement at the workshop. Diagnosis found the technician used the conventional pedal-bleeding method, letting air into the IPB. The IPB-specific bleeding procedure (requires scan tool to cycle the motor and build pressure) failed because pressure wouldn't hold. Disassembly revealed air entering had damaged the pressure sensor diaphragm, causing signal drift. Replaced the IPB module and properly bled the system using a Pressure Bleeder to resolve the fault.
BYD DTC AI Analysis

Tang DM-p IPB Internal Short After Water Ingress

✓
The vehicle had waded through water deeper than the wheel hub centre. The next morning on startup it threw C055F00 and multiple communication faults. Inspection found the IPB mounted at the firewall in the engine bay with a perished housing seal allowing water ingress. Stripping the IPB revealed obvious water staining on the internal PCB; the master cylinder pressure sensor solder joints had corroded, causing the signal line to short to ground. Sensor resistance measured only 200Ω (normal 2kΩ). Replaced the IPB assembly, waterproofed the wiring harness connector, and fitted a new seal. Fault cleared. Recommend checking the brake system promptly after wading.
BYD DTC AI Analysis

Han EV: Hydraulic pump abnormal noise during winter cold start with fault code

✓
During cold winter morning starts in northern regions, the IPB hydraulic pump ran continuously for over 5 seconds with excessive noise, then set C055F00. Diagnosis revealed low temperatures increased brake fluid viscosity, while worn hydraulic pump bearings reduced pressure-build efficiency. The system triggered the fault code after three consecutive failed self-tests. Replaced the IPB internal hydraulic pump motor (some dealers can replace the motor assembly separately instead of the entire unit) and switched to low-viscosity DOT4 brake fluid. Cold-start testing was normal and the fault did not reoccur.
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.