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) — Atto 8

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.

The C000400 fault code on the Atto 8 indicates a medium-severity electrical fault in TCS Control Valve B within the IPB brake system. It is usually caused by an open or shorted solenoid valve coil, or poor internal wiring contact in the IPB assembly. This can disable traction control and may reduce ESP support. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Open circuit or inter-turn short circuit in TCS solenoid valve B coil inside the IPB assembly (internal electrical fault)
  • 2Poor contact or broken solder joint in the internal wiring harness between the IPB and the solenoid valve.
  • 3Severe brake fluid contamination or crystallization causes the valve spool to stick, triggering abnormal drive current.
  • 4Damaged Valve Driver IC (solenoid valve driver chip) inside the IPB control unit.
  • 5Abnormal 12V power supply system voltage (surge or undervoltage) triggers a valve drive circuit protective fault.
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 conditions permit, remove the IPB and measure the internal TCS valve B coil resistance (standard value is typically 2-5Ω; refer to the workshop manual for exact specifications). Check for an open circuit or short to ground.
  • 4
    Check the brake fluid condition. If dark, emulsified, or water content >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 recurs.
  • 6
    If diagnostics confirm 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

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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 Atto 8 Fault Codes
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.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.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.