AU
C000404

DTC C000404 indicates an electrical fault in High Pressure Switch Valve 2 inside the ESP (Electronic Stability Program) hydraulic modulator — Seal U

Braking System

DTC C000404 indicates an electrical fault in High Pressure Switch Valve 2 inside the ESP (Electronic Stability Program) hydraulic modulator.

This solenoid valve, a key actuator in the ESP hydraulic unit, switches the brake fluid pressure path from the master cylinder to the wheel cylinder during ESP intervention.

The ESP ECU detects an anomaly in the valve drive circuit, such as an open solenoid coil, a short to power or ground, drive current exceeding the threshold (normal resistance is 2-8 Ω), or a damaged H-bridge drive circuit inside the ECU.

This fault disables the ESP, ABS, and TCS functions, but basic hydraulic braking remains functional.

This is a moderate safety-related fault.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal solenoid valve coil of the ESP hydraulic modulator assembly burned out or mechanically binding: Prolonged use or brake fluid contamination causes the valve spool to seize; the resulting abnormal drive current triggers the fault code.
  • 2Wiring harness and connector fault: Oxidation, loose connections, or water ingress at hydraulic modulator pin 25 (solenoid valve power supply) or pin 38 (ground), causing excessive contact resistance or intermittent open circuit.
  • 3Power supply system fault: Low battery charge, unstable DC-DC converter output (outside 9-16 V range), or corroded ground wire, causing insufficient or excessive solenoid drive voltage.
  • 4ESP ECU internal drive circuit damaged: Power MOSFET or driver chip breakdown prevents on/off control of High-Pressure Switching Valve 2.
  • 5System overheat protection triggered: Excessive brake fluid temperature caused by continuous aggressive driving, or degraded brake fluid (water content >3%), triggers a thermal protective shutdown of the solenoid valve.
What To Do
  • 1
    Use the BYD VDS/ED400 diagnostic tool to read the complete fault codes, confirm C000404 is a current fault rather than a history fault, and check for accompanying related faults such as C080001 (low voltage) or C007204 (overtemperature).
  • 2
    Basic power supply check: Measure battery static voltage (>12V) and startup voltage. Check DC-DC output stability. Inspect the ABS 25A fuse and ESP ground wire (G101/G102) connections for oxidation or looseness.
  • 3
    Harness continuity check: Disconnect the ESP hydraulic modulator connector. Measure the resistance between the Solenoid Valve 2 power supply pin (usually Pin 25) and the ground pin (Pin 38). Standard value: 2–8 Ω. Check harness insulation to ground and power supply to rule out a short circuit.
  • 4
    Component active test: Perform the 'solenoid valve actuation test' using the diagnostic tool and listen for a clear 'click' from High-Pressure Switching Valve 2. If there is no sound or the current is >2A, diagnose a fault in the valve body or ECU drive circuit.
  • 5
    Cross-check: If conditions permit, install the faulty vehicle's ESP hydraulic modulator on a known good vehicle for testing, or swap a known good modulator onto the faulty vehicle. This determines whether the fault is in the valve body or the ECU.
  • 6
    Replacement: After confirming an internal hydraulic modulator fault, replace the ESP hydraulic modulator assembly with ECU (part number depends on vehicle model, e.g., E5/Qin/Song). Do not disassemble or repair components separately.
  • 7
    System recovery: After replacement, perform the brake line bleeding procedure (use the diagnostic tool to drive the pump motor to bleed the system). Clear the fault code. Drive in a straight line at speeds above 30 km/h for 10 seconds to complete sensor calibration. Finally, perform an ABS/ESP dynamic road test to verify the repair.
Real-World Cases
BYD DTC AI Analysis

BYD Qin Pro ESP hydraulic modulator solenoid valve body damaged

✓
A 2018 BYD Qin Pro DM with 80,000 km had the ESC warning light constantly on. The scan tool read fault code C000404 (High Pressure Switch Valve 2 fault), which would not clear. Battery voltage was normal. Solenoid valve 2 resistance at the hydraulic modulator connector measured infinite (open circuit), while other solenoids measured 4.2Ω. Disassembled the unit and found the internal solenoid coil burned out, with a burnt smell. Replaced the ESP hydraulic modulator assembly with integrated ECU (part number SA-3636100), bled the system and ran calibration. Fault cleared.
Original source ↗
BYD DTC AI Analysis

BYD Song DM brake fluid overheating causes intermittent C000404 fault

✓
2019 BYD Song DM. Customer reported the dashboard displayed "Check ESP System" after aggressive high-speed driving; the fault cleared after restarting. Scan tool showed history codes C000404 and C007204 (system overheat). Brake fluid moisture was 4% and fluid was black. During repeated braking tests, hydraulic modulator surface temperature exceeded 120°C. Replaced DOT4 brake fluid and flushed the system; the fault has not returned. Reduced boiling point of aged brake fluid caused vapor lock and high temperatures during continuous braking, triggering solenoid thermal protection.
Original source ↗
BYD DTC AI Analysis

BYD E5 Battery Drain Causes ESP Solenoid Valve Power Supply Fault

✓
A 2019 E5 taxi came in one week after battery replacement with the ESP warning light on and DTC C000404 stored. Idle battery voltage measured only 11.8V; DC-DC output was unstable, fluctuating between 12–14V. With headlights and AC on, ESP supply voltage dropped to 9.2V, below the solenoid valve operating threshold. Replaced the faulty DC-DC controller. Checked the ESP ground wire and found corrosion at the ground point. Cleaned and tightened the connection to stabilize voltage, cleared the fault codes, and confirmed the fix.
Original source ↗
BYD DTC AI Analysis

Oxidised wiring harness connector on BYD Qin caused intermittent high-pressure switch valve fault

✓
2017 BYD Qin. Drove through standing water; ESP warning light came on intermittently. Scan tool showed intermittent fault code C000404. Found green corrosion inside the ESP hydraulic modulator 25-pin connector. Pin 25 (solenoid valve power supply) had backed out slightly. Cleaned the connector with electronic cleaner, applied conductive paste, and repaired the pin retention with a special tool. Reconnected and ran a water spray test and rough road test; the fault did not recur. Poor connector sealing allowed water ingress, causing increased contact resistance.
Original source ↗
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.