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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
Real-World Cases
BYD DTC AI Analysis

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

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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

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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

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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

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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
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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.