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C055F92

DTC C055F92 indicates an IPB (Integrated Power Brake) hydraulic subsystem performance fault — Seal U

Braking System

DTC C055F92 indicates an IPB (Integrated Power Brake) hydraulic subsystem performance fault.

This DTC indicates the IPB control unit detects abnormal hydraulic circuit pressure, excessive pressure build-up time, insufficient pressure holding capability, or abnormal hydraulic pump motor operation.

The IPB system integrates electric vacuum assist, ESP electronic stability control, and ABS anti-lock braking functions.

This fault may reduce brake assist, harden the brake pedal, restrict or disable active safety functions (AEB/ACC), or in extreme cases, trigger the brake system downgrade protection mode.

The fault suffix '92' indicates a hydraulic circuit performance deviation or intermittent operating abnormality rather than a complete failure.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Worn or seized IPB internal hydraulic pump motor causing insufficient pressure build-up or delayed response; typically occurs in high-mileage vehicles or results from internal corrosion due to prolonged failure to replace brake fluid.
  • 2Insufficient brake fluid, fluid degradation, or vapor lock, with the fluid level below the minimum mark or water content exceeding 3%, causing abnormal hydraulic compressibility.
  • 3A sticking solenoid valve (inlet/outlet valve) or aged sealing ring inside the IPB control unit causes abnormal pressure retention or internal leakage.
  • 4Wheel speed sensor signal drift or wiring interference causes the IPB to miscalculate wheel cylinder pressure demand, triggering abnormal hydraulic system regulation.
  • 5Unstable IPB power supply (poor contact at the 15A fuse) or momentary CAN bus communication interruption, causing hydraulic control command execution failure.
What To Do
  • 1
    Use VDS2000 or the BYD dedicated diagnostic tool to read the complete fault codes and freeze frame data. Record key parameters at the time of the fault, such as vehicle speed, master cylinder pressure, and motor current. Check for accompanying wheel speed-related faults such as C003F and C0040.
  • 2
    Visually inspect the brake fluid reservoir level. Check the IPB assembly, four-wheel brake calipers, and lines for leaks. If necessary, perform a pressure holding test (pressure drop must be <10 bar over 10 minutes).
  • 3
    Measure the IPB assembly power supply voltage (12.5-14.5 V), ground resistance (<1 Ω), and CAN-H/CAN-L waveforms (2.5 V reference, dominant level 3.5 V/1.5 V). Check the terminals of the 30-pin IPB connector for backed-out pins or oxidation.
  • 4
    Perform the IPB system bleeding procedure: Use the diagnostic tool to activate bleeding mode, bleed in the sequence of right rear - left rear - right front - left front, and verify no air bubbles remain and pedal travel is normal.
  • 5
    Check brake fluid quality using a brake fluid moisture tester. If the moisture content is >3% or the fluid is cloudy, completely replace the DOT4 brake fluid and flush the system.
  • 6
    If the above checks are normal, flash the IPB software (version must be ≥2023-Q2). After flashing, clear the fault codes and perform a 30-minute road test (including an emergency braking test above 40 km/h).
  • 7
    If the fault recurs, replace the IPB integrated electro-hydraulic module assembly. After replacement, use the diagnostic tool to perform coding matching, longitudinal acceleration sensor calibration, and ESP function learning.
Real-World Cases
BYD DTC AI Analysis

Aging IPB pump motor in Song PLUS DMi caused intermittent pressure build-up failure

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82,000 km. Owner reported the instrument cluster intermittently displaying "Braking System Fault," with the brake pedal going hard when the warning light illuminated. The scan tool retrieved C055F92 and C055E00 (hydraulic pump motor fault). Freeze frame data showed 120 bar master cylinder pressure demand against 68 bar actual, with motor current spiking to 18 A (normal <12 A). Disassembled the IPB and found the motor carbon brushes worn beyond limits and the commutator scored. Replaced the IPB assembly and reflashed the software. Fault resolved.
BYD DTC AI Analysis

Brake fluid leak caused insufficient hydraulic system pressure

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2021 Song PLUS DM-i. ESC and ABS warning lights lit up simultaneously while driving. Inspection found slight seepage at the left rear wheel cylinder and brake fluid below MIN. Low fluid prevented the IPB accumulator from building adequate pressure, triggering C055F92. Replaced the left rear wheel cylinder repair kit and brake fluid, bled the system, and cleared the fault code. Advised the owner to replace brake fluid every 2 years to prevent recurrence.
BYD DTC AI Analysis

Wheel speed sensor signal interference caused hydraulic system false alarm

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Vehicle frequently set DTC C055F92 when driving on rough roads; operated normally on smooth surfaces. Inspection revealed the right front wheel speed sensor harness was rubbing against the steering knuckle, with worn insulation causing an intermittent short. When the IPB detected sudden wheel speed signal fluctuations, it misinterpreted these as wheel lock-up and repeatedly adjusted hydraulic pressure, eventually triggering a hydraulic system performance fault. Repaired the harness and secured its routing; the fault has not returned.
BYD DTC AI Analysis

IPB software calibration error causing cold-start fault

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In -10°C winter temperatures, DTC C055F92 appeared every cold start and cleared after 10 minutes driving. Diagnosis revealed outdated IPB software with a defective pressure sensor zero-drift compensation algorithm for low temperatures. Upgraded IPB software to IPB_3.2.1_CN via VDS2000 and performed sensor zero-point calibration. Fault resolved.
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.