AU
C000404

DTC C000404 indicates an electrical fault in High Pressure Switch Valve 2 inside the ESP (Electronic Stability Program) hydraulic modulator — Atto 8

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.

The C000404 fault code on the Atto 8 indicates an electrical fault in High Pressure Switch Valve 2 inside the ESP hydraulic modulator. This medium-severity issue can disable ESP, ABS, and traction control, though basic braking should still work. Common causes include a burnt or stuck solenoid valve, or wiring/connector faults from corrosion, looseness, or water ingress. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Internal solenoid valve coil in the ESP hydraulic modulator assembly burnt out or mechanically stuck: Prolonged use or brake fluid contamination causes the valve spool to seize, resulting in abnormal drive current that triggers the fault code.
  • 2Wiring harness and connector fault: Oxidation, loose connection, 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 the 9-16V range), or ground wire corrosion, causing solenoid drive undervoltage or overvoltage.
  • 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: Continuous aggressive driving causes excessive brake fluid temperature, or degraded brake fluid (water content >3%) causes 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, not 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 cranking voltage. Check DC-DC output stability. Inspect the ABS 25A fuse and ESP ground wires (G101/G102) to verify connections are secure and free of oxidation.
  • 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: Use the diagnostic tool to perform the 'solenoid valve actuation test' and listen for a clear 'click' from High-voltage Switching Valve 2. If there is no sound or the current is >2A, the valve body or ECU drive circuit is faulty.
  • 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 to verify. This determines whether the valve body or the ECU is faulty.
  • 6
    Replacement: After confirming an internal hydraulic modulator fault, replace the ESP hydraulic modulator assembly with ECU (verify spare part number by vehicle model, e.g., E5, Qin, Song). Do not disassemble for separate repair.
  • 7
    System restoration: After replacement, bleed the brake lines (use the diagnostic tool to drive the pump motor for bleeding), clear the fault codes, drive in a straight line at over 30 km/h for 10 seconds to complete sensor calibration, and finally perform an ABS/ESP dynamic road test for verification.
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 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.