AU
C000304

DTC C000304 indicates an electrical or functional fault in High Pressure Switch Valve 1 inside the ESP (Electronic Stability Program) hydraulic modulator — Atto 8

Braking System

DTC C000304 indicates an electrical or functional fault in High Pressure Switch Valve 1 inside the ESP (Electronic Stability Program) hydraulic modulator.

This solenoid valve is a core actuator in the ESP Hydraulic Control Unit (HCU).

It controls the brake fluid pressure build-up path from the master cylinder to the wheel cylinders during ABS/ESP intervention.

The ECU sets this fault upon detecting an open circuit or short circuit (to ground or power) in the valve drive circuit, or an abnormal valve spool response.

When this fault occurs, the ESP system enters fail-safe mode, disables active intervention functions, retains only conventional hydraulic braking, and illuminates the ABS/ESP warning light on the instrument cluster.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Open circuit, short circuit, or resistance drift (outside the standard 5-20Ω range) in the High-Pressure Switching Valve 1 solenoid coil inside the ESP hydraulic modulator assembly
  • 2Poor contact between the hydraulic modulator and wiring harness connector, backed-out terminals, oxidation, corrosion, or waterproofing failure interrupting signal transmission.
  • 3ESP ECU internal solenoid valve drive circuit fault, such as a burnt H-bridge driver chip, MOSFET, or associated sampling resistor.
  • 4Severe brake fluid contamination (excessive water content or crystallized impurities) causes mechanical binding of the valve spool, triggering abnormal position sensor feedback.
  • 5Abnormal system power supply voltage: battery voltage below 9V or above 16V causes the ECU to misjudge the solenoid valve drive status.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS/VDS2000/VDCI) to read the fault codes and freeze frame data. Record operating parameters such as vehicle speed and voltage at the time of the fault. Clear the fault codes and perform a road test to reproduce the fault.
  • 2
    Check that the brake fluid level is between the MAX and MIN marks. Inspect the brake fluid for discoloration, cloudiness, or metal debris. Replace the brake fluid if necessary.
  • 3
    Disconnect the ESP hydraulic modulator electrical connector and check the pins (typically pin 25 power, pin 38 ground, and related signal pins) for oxidation, backed-out pins, or corrosion. Clean with electrical contact cleaner and measure the supply voltage (standard: 10-16 V).
  • 4
    Measure the solenoid coil resistance of High Voltage Switching Valve 1 (refer to the vehicle workshop manual; typically 5-20 Ω). If the resistance is infinite (open circuit) or close to 0 Ω (short circuit), confirm an internal fault in the valve body.
  • 5
    Connect the diagnostic tool and perform the 'Solenoid Valve Active Test' (component test). Listen for an actuation 'click' from the corresponding valve body inside the hydraulic modulator. If the sound is absent or faint and circuit measurements are normal, confirm mechanical binding of the valve body or coil failure.
  • 6
    If the above checks are normal but the fault code persists, check the continuity and insulation of the wiring harness from the ECU to the valve body. After ruling out harness wear and shorts to ground, the ECU internal drive circuit is faulty. Replace the ESP hydraulic modulator assembly with ECU.
  • 7
    After replacing the hydraulic modulator, perform the brake system bleeding procedure (use a diagnostic tool to activate the pump motor for bleeding). Calibrate the steering angle sensor and G-sensor. Finally, drive in a straight line at a speed above 30 km/h for at least 10 seconds to complete the system self-check.
Real-World Cases
BYD DTC AI Analysis

BYD Qin ESP hydraulic modulator high-pressure switch valve internal open circuit

✓
Dashboard ABS/ESP warning light constantly illuminated. Brake pedal feels normal but ESP intervention inactive. Scanned with VDS and retrieved DTC C000304 (High pressure switching valve 1 fault). Brake fluid normal. Measured 12.4V supply at hydraulic modulator connector pin 25; ground at pin 38 normal. Disconnected connector and measured infinite resistance on high pressure switching valve 1 coil (normal approx 8Ω), indicating internal coil open circuit. Replaced ESP hydraulic modulator assembly with integrated ECU. Performed brake bleed and calibration; fault cleared. Road tested above 30 km/h, system self-check passed.
Original source ↗
BYD DTC AI Analysis

Abnormal power supply voltage causes false valve fault codes on BYD E5

✓
The E5 displayed C000304 and multiple solenoid fault codes simultaneously. Inspection found an aftermarket high-power audio system installed. Battery voltage fluctuated severely between 11V and 14.5V during night driving. Measuring the ESP module power supply revealed unstable alternator output; coupled with battery aging (increased internal resistance), this caused voltage to drop below 8.5V when the ECU drove the high-pressure switching valves, triggering the fault codes. Replaced the alternator rectifier and battery, checked body ground points, cleared the codes. No recurrence on road test.
Original source ↗
BYD DTC AI Analysis

Crystallised brake fluid caused the high-pressure switch valve to stick.

✓
The vehicle waded through water without replacing the brake fluid. Two years later, DTC C000304 set. Disassembling the ESP hydraulic modulator revealed the high-pressure switch valve 1 core stuck in the normally closed position by white crystalline deposits—salts from corrosion after the brake fluid absorbed moisture. During active testing with the scan tool, no valve actuation sound was heard, though coil resistance measured normally at 6.5Ω. The valve body is non-serviceable, so we replaced the hydraulic modulator assembly. After thoroughly flushing the brake lines and replacing the DOT4 brake fluid, the fault resolved. Advise the owner to replace the brake fluid every 2 years or 40,000 km.
Original source ↗
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.