AU
C000304

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

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.

The C000304 fault code on the Seal U indicates a medium-severity fault with High Pressure Switch Valve 1 inside the ESP hydraulic modulator. It may be caused by an open or short circuit in the solenoid coil, incorrect resistance, or poor/corroded connector contact. The ESP may disable active stability and ABS functions, leaving normal braking only. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 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, terminal back-out, 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 related sampling resistor.
  • 4Severe brake fluid contamination (excessive water content or crystallised impurities) causes mechanical sticking of the valve spool, triggering abnormal position sensor feedback.
  • 5Abnormal system supply voltage: battery voltage below 9V or above 16V causes the ECU to misinterpret the solenoid valve drive status.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS/VDS2000/VDCI) to read the fault code and freeze frame data. Record environmental parameters at the time of the fault, such as vehicle speed and voltage. Clear the fault code and perform a road test to reproduce the fault.
  • 2
    Check if the brake fluid level is between the MAX and MIN marks. Check if the brake fluid is discolored, cloudy, or contains metal debris. Replace the brake fluid if necessary.
  • 3
    Disconnect the ESP hydraulic modulator electrical connector. Inspect the pins (typically power pin 25, ground pin 38, and related signal pins) for oxidation, pushback, or corrosion. Clean the pins with electrical contact cleaner and measure the supply voltage (standard: 10-16 V).
  • 4
    Measure the solenoid coil resistance of High-pressure Switching Valve 1 (refer to the vehicle repair manual; typically 5-20 Ω). If the resistance is infinite (open circuit) or close to 0 Ω (short circuit), this indicates 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 a mechanically stuck valve body or failed coil.
  • 6
    If the fault code persists after all above checks, check the continuity and insulation of the wiring harness between the ECU and the valve body. If the harness is not worn or shorted 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 trigger pump motor 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 Seal U Fault Codes
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.