AU
C000400

DTC C000400 indicates an electrical fault in TCS (Traction Control System) Control Valve B within the IPB (Intelligent Integrated Braking System) (Fault Type 2 typically designates an open circuit, short circuit, or abnormal current) — Seal U

Braking System

DTC C000400 indicates an electrical fault in TCS (Traction Control System) Control Valve B within the IPB (Intelligent Integrated Braking System) (Fault Type 2 typically designates an open circuit, short circuit, or abnormal current).

In the BYD One-Box brake-by-wire system, the IPB integrates the ESP function.

TCS Valve B typically identifies the solenoid valve controlling rear wheel brake hydraulic pressure (or a specific channel valve).

This fault means the IPB ECU detects the Valve B drive circuit current falls outside the specified threshold (too high or too low).

Causes include an open or shorted coil, or mechanical binding of the valve spool resulting in abnormal drive current.

This fault disables the TCS function, preventing the system from actively applying braking force to control the slip ratio during drive wheel slip.

Under extreme conditions, the system downgrades the ESP function, retaining only basic ABS capabilities.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Open circuit or inter-turn short circuit in TCS solenoid valve B coil inside IPB assembly (internal electrical fault)
  • 2Poor contact in the IPB-to-solenoid valve internal wiring harness, or detached solder joint.
  • 3Severely contaminated or crystallized brake fluid causes the valve spool to stick, resulting in abnormal drive current.
  • 4Damaged internal Valve Driver IC (solenoid valve driver chip) in the IPB control unit.
  • 5Abnormal 12V power supply system voltage (surge or undervoltage) triggers a protective fault in the valve drive circuit.
What To Do
  • 1
    Use VDS or Launch X431 to read all fault codes, confirm whether the C000400 status is current or history, and check for related faults such as C0003XX/C0005XX.
  • 2
    Check the IPB wiring harness connectors (especially the 24-pin main connector) for backed-out pins, corrosion, or signs of water ingress. Measure the terminal contact resistance.
  • 3
    If possible, remove the IPB and measure the internal TCS valve B coil resistance (standard value is typically 2-5 Ω; refer to the repair manual for exact specifications). Check for an open circuit or short to ground.
  • 4
    Check the brake fluid condition. If darkened, emulsified, or water content is >3%, replace the brake fluid and perform an IPB cleaning test.
  • 5
    Attempt to clear the fault code and perform a TCS function test: drive on a slippery road surface or raise the vehicle to simulate drive wheel slip, and observe if the fault reoccurs.
  • 6
    If testing confirms a hardware fault, replace the IPB assembly (part number reference: Song PLUS DM-i typically uses the 6A-3630100 series). Do not disassemble or repair the internal hydraulic unit.
  • 7
    After replacement, perform: brake line bleeding → IPB calibration (pedal stroke sensor calibration) → longitudinal acceleration sensor calibration → road test to verify TCS function
Real-World Cases
BYD DTC AI Analysis

Song PLUS DM-i IPB internal solenoid valve coil short circuit

✓
At 30,000 km, the ABS/ESC warning light stayed on after driving through standing water in the rain. The scan tool read active fault code C000400. Resistance between the IPB connector TCS valve B drive pin and ground measured 0.8Ω (specification >2Ω), indicating an internal coil short. Disassembled the IPB and found the hydraulic control unit seal had deteriorated, allowing water in. Replaced the IPB assembly (part number 6A-3630100C), bled the brakes and calibrated the sensors. Fault resolved.
BYD DTC AI Analysis

Loose IPB wiring harness connector causing intermittent fault

✓
When driving on rough roads, the ESC warning light illuminates intermittently; the vehicle operates normally on smooth surfaces. Read DTC C000400 as an intermittent fault. Found pin 16 of the IPB assembly 24-pin connector (TCS valve B drive line) retracted with oxidation on the contact surface. Repaired the harness terminal, applied conductive grease, and installed a harness clip to prevent vibration-induced loosening. Fault has not recurred.
BYD DTC AI Analysis

Brake fluid contamination caused TCS Valve B to stick

✓
Improper maintenance with non-DOT4 brake fluid caused severe emulsification and degradation after 2 years. The owner reported severe wheel slip during acceleration (TCS failure). Diagnostics revealed DTCs C000400 and C001100 (hydraulic unit faults). Disassembling the IPB revealed the TCS valve B spool jammed with sludge and solenoid current abnormal. Fix: Thoroughly flushed the brake lines, replaced the IPB assembly and all brake fluid. Fault resolved.
BYD DTC AI Analysis

Resolved IPB false fault alarm via software update

✓
2021 Song PLUS DM-i logged C000400 at 1000 km. TCS tested normal with no braking anomalies. Found IPB software ran early version V1.02 with a bug that set the solenoid valve drive current threshold too sensitive. Fix: Upgraded IPB ECU software to V1.05, optimized solenoid valve diagnostic strategy, cleared fault codes and monitored for one month with no recurrence.
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.