AU
C000204

This DTC indicates an electrical fault (typically an open circuit or abnormal resistance) in Circuit Control Valve 2 inside the ESP (Electronic Stability Program) hydraulic modulator — Atto 8

Braking System

This DTC indicates an electrical fault (typically an open circuit or abnormal resistance) in Circuit Control Valve 2 inside the ESP (Electronic Stability Program) hydraulic modulator.

During active ESP braking intervention, this solenoid valve precisely controls the build-up and release of hydraulic pressure in a specific brake circuit (typically the rear wheel brake circuit).

When this fault triggers, the ESP system enters degraded mode and disables electronic stability control, traction control (TCS), and automatic emergency braking (AEB).

The vehicle typically retains conventional hydraulic braking.

This is a hardwired circuit fault, not a software false positive.

Focus inspection on the solenoid valve coil integrity and the ECU driver circuit.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1ESP hydraulic modulator assembly internal solenoid valve coil open circuit or resistance out of specification (normal range is typically 12-25Ω; refer to the vehicle repair manual for the specific model).
  • 2Oxidized, loose, or water-corroded pins in the hydraulic modulator wiring harness connector (usually located on the top or side of the modulator), causing an open circuit.
  • 3Damaged ABS/ESP ECU internal valve body drive circuit (e.g., shorted or open power MOSFET), unable to supply operating current to the valve.
  • 4Power supply circuit fault, including a blown dedicated ABS system fuse (usually 30A-40A) or poor contact in the power wiring harness.
  • 5Poor or broken solder joints on the hydraulic modulator internal flexible printed circuit board (FPC) or coil pins
What To Do
  • 1
    Use the BYD VDS2000/VDS3000 diagnostic tool to read the complete fault code data stream and freeze frame data. Confirm vehicle speed, voltage, and other operating conditions at the time of the fault to rule out intermittent faults.
  • 2
    Disconnect the battery negative terminal, remove the ESP hydraulic modulator assembly (located at the front left or front right of the engine compartment, with brake lines), and inspect the exterior for impact damage, leaks, or burn marks.
  • 3
    Measure the resistance of the corresponding pin on the hydraulic modulator wiring harness connector (refer to circuit diagram E5, typically the valve body drive pin) and compare it with the standard value (approximately 12-25Ω). An infinite resistance confirms an open circuit in the valve body coil.
  • 4
    Check wiring harness continuity: Measure continuity of the wiring harness from the ECU to the valve body. Check for open circuits, shorts to ground, or shorts to power. Focus on the firewall wiring grommet and connector bends.
  • 5
    Perform valve body active test: Reconnect the diagnostic tool. With the ignition switch in the ON position, perform the 'Circuit Control Valve 2' active test. Listen for a clear clicking engagement sound. Use an oscilloscope to observe the drive current waveform (normally a sawtooth wave with a peak of approximately 1.5-2A).
  • 6
    If circuit measurements are normal but the valve body does not operate, replace the ESP hydraulic modulator assembly with ECU (refer to the E5 brake system BOM for the part number). After replacement, perform the brake system bleeding and sensor calibration procedures.
  • 7
    Clear the fault code and perform a road test (including straight-line acceleration, emergency braking, and cornering) to confirm the ESP function operates normally and the system generates no new fault codes.
Real-World Cases
BYD DTC AI Analysis

High-frequency use caused the valve body coil to age and develop an open circuit in the E5 taxi.

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Vehicle: 2018 BYD E5, 280,000 km. Symptoms: ABS and ESP warning lights on constantly. Retrieved fault code C000204. Diagnosis: Measured resistance between hydraulic modulator harness connector pins B05-12 and B05-13 (circuit control valve 2 drive terminals): open circuit (specification 15Ω). Confirmed coil break inside valve body. Disassembled hydraulic modulator; internal solenoid coil magnet wire burned out from overheating due to prolonged high-frequency operation. Solution: Replaced ESP hydraulic modulator assembly with integrated ECU (part number: C000204-compatible model). Bled brakes and calibrated yaw rate sensor after replacement. Fault resolved.
BYD DTC AI Analysis

Driving through water corroded the wiring harness connector, causing intermittent faults.

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Model: 2019 BYD E5. Symptoms: ESP warning light illuminates intermittently after driving in rain; clears after restarting. DTC C000204 stored as a history code. Diagnosis: Valve body resistance normal in dry conditions. Simulated water spray test showed fluctuating resistance between pins 9 and 10 of connector B05 (0.5–50Ω). Disassembled connector and found degraded seal and green copper oxide on pin surfaces. Repair: Cleaned harness connector pins, applied electronic-grade protective silicone grease, replaced seal, and re-secured waterproof cover. Follow-up: Monitored for one month; no recurrence.
BYD DTC AI Analysis

ECU internal driver circuit damage prevented the valve from energizing.

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Vehicle: 2018 BYD E5. Issue: C000204 DTC set after brake fluid replacement. Diagnosis: External harness continuity normal; valve body resistance 14.5Ω (normal); power supply 12.5V (normal). During active testing, oscilloscope showed no drive voltage waveform at the ECU output. ABS ECU internal valve body driver chip damaged—likely from backflow impact during improper brake bleeding. Fix: ECU and hydraulic modulator are integrated. Replaced ESP hydraulic modulator assembly. Fault cleared. Note: Follow brake fluid replacement specifications strictly. Avoid rapidly pumping the brake pedal to prevent hydraulic shock.
Other Atto 8 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.