AU
C059400

DTC C059400 indicates a functional fault in the brake booster motor within the IPB (Intelligent Power Brake) system — Seal U

Braking System

DTC C059400 indicates a functional fault in the brake booster motor within the IPB (Intelligent Power Brake) system.

This brushless DC motor integrates into the IPB electro-hydraulic module to replace the traditional vacuum booster.

It provides electric brake assist based on pedal input and coordinates energy recovery.

The IPB control unit sets this code upon detecting abnormal motor current, abnormal speed feedback, a drive circuit fault, or motor mechanical binding.

This fault causes a hard brake pedal, reduced brake assist, and increased braking distance.

It also degrades or disables functions including ABS, ESC, and Automatic Emergency Braking, severely compromising driving safety.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal IPB brake booster motor damage: motor bearing wear, rotor seizure, or permanent magnet demagnetization preventing normal motor operation or causing abnormal current.
  • 2Motor drive circuit fault: Damaged MOSFET, gate driver chip, or sampling resistor in the internal IPB H-bridge drive circuit causes an abnormal motor drive signal.
  • 3Abnormal power supply: Poor contact in the IPB module terminal 30 constant power or terminal 31 ground circuit, loose fuse connection, or voltage fluctuation (below 9V or above 16V), causing unstable motor power supply.
  • 4Motor position sensor fault: Abnormal signal from the Hall sensor or resolver integrated into the motor prevents the control unit from accurately identifying the motor rotor position.
  • 5Control software or calibration data fault: Internal software bug in the IPB control unit, corrupted calibration data, or outdated software version causes incorrect assessment of motor status.
What To Do
  • 1
    Use the dedicated diagnostic tool (VDS2000/3000) to read the complete fault codes and freeze frame data. Check specifically for accompanying fault codes such as C059100 (motor voltage too low), C059500 (internal drive fault), or C003700 (pump motor fault). Record parameters such as vehicle speed and pedal travel at the time of the fault.
  • 2
    Visually inspect the IPB electro-hydraulic module for damage, fluid leaks, or burn marks. Check the master cylinder connection and wiring harness connectors (especially the large 30pin connector) for looseness, water ingress, or corrosion. Measure the voltage between the power supply (Pin30) and ground (Pin31) to verify it is within 12V±0.5V.
  • 3
    Perform the IPB function self-test: Use the diagnostic tool to run the brake booster motor drive test. Listen to the motor operating sound (it should run evenly without binding), observe if the brake pedal sinks automatically, and simultaneously measure the motor operating current using a current clamp (normal value: 5-15A; abnormal values exceed 25A or approach 0A).
  • 4
    Check the brake system hydraulic circuit: Verify the brake fluid level is normal and the lines have no leaks. Rule out indirect faults where excessive hydraulic resistance overloads the motor.
  • 5
    Software check and update: Verify the IPB control unit software version against the BYD Technical Service Bulletin (TSB). If an optimized software version exists for this fault code, perform an online programming update. Perform IPB recalibration and zero-point learning if necessary.
  • 6
    If all above checks are normal but the fault persists, this confirms a hardware fault in the IPB internal motor or drive circuit. Replace the IPB electro-hydraulic module assembly (verify the part number matches the vehicle configuration). After replacement, program the new module, bleed the brake system (use a diagnostic tool to execute the bleeding procedure), and calibrate the ESC.
Real-World Cases
BYD DTC AI Analysis

Song Plus DM-i: IPB motor jamming causes stiff brake pedal

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Vehicle had covered 30,000 km. After a cold start one morning, the dashboard displayed "Brake System Fault" with DTC C059400 stored. The driver reported the brake pedal had gone noticeably hard, requiring heavy pedal effort to slow the vehicle. Visual inspection of the IPB module showed no external damage. During the motor drive test, a clicking sound came from inside but the motor did not turn. Disassembling the IPB revealed the internal assist motor bearing had corroded and seized (the vehicle had previously waded through water). Replaced the IPB assembly and performed the brake bleed procedure to clear the fault.
BYD DTC AI Analysis

Loose ground wire caused intermittent C059400 fault

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ABS/ESC warning light occasionally illuminated when driving over speed bumps or rough roads. Scan tool retrieved intermittent DTC C059400. While checking IPB supply voltage, found the left engine bay ground point (G101) bolt loose. This caused excessive IPB ground resistance (measured 2.3Ω; normal is <0.1Ω). Poor grounding during bumps caused momentary voltage loss to the motor drive. Tightened the ground point and applied conductive paste. Road-tested 50 km continuously — fault did not recur.
BYD DTC AI Analysis

Outdated software caused a false brake booster motor fault.

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Vehicle had 1000 km when it suddenly threw a braking system fault at highway speed: DTC C059400. Checked motor and wiring harness—both normal. Voltage stable at 12.4V. Tech bulletin search revealed IPB software V1.02.03 on this batch has a bug: motor current sampling threshold set too tight, causing false triggers during rapid deceleration when switching between regen and mechanical braking. Flashed IPB to V1.03.05. Fault cleared, no recurrence after one month of monitoring.
BYD DTC AI Analysis

Brake fluid replaced without bleeding, causing motor overload damage

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The vehicle developed a C059400 fault after brake fluid replacement at a non-authorised workshop. Investigation revealed the technician used the traditional foot-pumping method to bleed the brakes rather than performing the IPB active bleed procedure with a diagnostic tool. Air entered the wheel cylinders, causing the IPB motor to run under high load for an extended period while attempting to build pressure, eventually overheating and failing. Motor winding resistance measured open circuit (normal approximately 1.5Ω). Replaced the IPB module and correctly performed the diagnostic tool bleed procedure to resolve.
BYD DTC AI Analysis

Burnt IPB driver chip caused motor failure

✓
Vehicle failed to power up (READY light off) with multiple brake system fault codes present, primary code C059400. IPB power supply tested normal, but the scan tool could not drive the motor. Opened the IPB control unit board and found visible burn marks on the H-bridge driver chip (typically INFINEON or NXP), along with characteristics of a motor winding short. Determined that motor insulation aging caused a short circuit, overloading and burning out the driver chip. Replaced the IPB assembly (motor and drive board integrated); 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.