AU
C001904

This DTC indicates an electrical fault in the drive circuit or actuator body of the left rear wheel outlet solenoid valve (Dump Valve/Outlet Valve) inside the ABS/ESP hydraulic control unit (HCU) — Atto 3

Braking System

This DTC indicates an electrical fault in the drive circuit or actuator body of the left rear wheel outlet solenoid valve (Dump Valve/Outlet Valve) inside the ABS/ESP hydraulic control unit (HCU).

The outlet valve is a switching solenoid in the ABS pressure modulation unit that controls brake fluid return from the left rear wheel to the reservoir.

The ABS ECU drives it via a PWM signal.

The '04' sub-code in DTC C001904 typically indicates an open drive circuit, an open solenoid coil, or a short to ground.

This fault prevents the ABS/ESP system from reducing pressure at the left rear wheel.

During emergency braking or stability control, the left rear wheel may lock up or experience abnormal brake force distribution, severely compromising vehicle handling safety.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Open circuit, short circuit, or abnormal resistance in the left rear outlet valve solenoid coil inside the ABS hydraulic modulator assembly (normal range typically 20-60 Ω, depending on specific model)
  • 2ABS pump wiring harness connector (usually located near the engine compartment firewall): terminal pin back-out, oxidation, corrosion, or poor contact interrupting the drive signal.
  • 3Damaged ABS ECU internal solenoid valve drive circuit (such as MOSFET power transistor breakdown or driver chip failure), unable to provide approximately 5-6A of drive current.
  • 4Open or short circuit in the internal wiring harness or mating connector between the ECU and hydraulic modulator (the hydraulic modulator and ECU are usually an integrated unit; internal ribbon cable fault).
  • 5In extreme cases, severe brake fluid contamination causes the valve spool to bind, triggering solenoid valve drive overload or abnormal position feedback (some advanced systems feature valve position monitoring).
What To Do
  • 1
    Step 1: Visual inspection and basic measurement. Disconnect the ABS hydraulic modulator assembly wiring harness connector and check the pins for corrosion or backing out. Use a multimeter to measure the left rear outlet solenoid valve resistance (refer to the specific vehicle repair manual for the pins, usually MR or ML+ and the corresponding Out terminal). The normal value is 20-60 Ω. If the resistance is infinite or significantly deviates from the standard value, the hydraulic modulator internal solenoid valve is faulty.
  • 2
    Step 2: Check wiring harness continuity and insulation. With the connectors disconnected, measure the continuity resistance of the drive circuit from the ECU to the solenoid valve (should be <1Ω), and measure the insulation resistance of this circuit to body ground and to power (should be >10MΩ). If a short or open circuit exists, repair the wiring harness.
  • 3
    Step 3: ECU drive capability verification (special equipment required). If the solenoid valve resistance is normal, use an oscilloscope or the actuator test function of a dedicated diagnostic tool to trigger the left rear outlet valve. Monitor the ECU terminal for a normal 12V square-wave drive signal and current draw (approximately 4-6A). A missing drive signal indicates an internal ABS ECU fault.
  • 4
    Step 4: Assembly replacement and matching. If the hydraulic modulator internal solenoid valve or ECU drive circuit is damaged, replace the ABS hydraulic modulator assembly (with ECU). After replacement, use the BYD VDS diagnostic tool to perform: ① the bleeding procedure (requires the dedicated fluid draining tool); ② wheel speed sensor signal verification; ③ lateral acceleration/yaw rate sensor calibration; ④ a road test to verify ABS function.
  • 5
    Step 5: Dynamic function verification. After clearing the fault code, perform an emergency braking test at 30-40km/h on a safe, low-adhesion road surface (such as wet asphalt) to confirm the ABS activates normally without abnormal noise. Simultaneously monitor the data stream to verify the left rear wheel speed signal drops normally, ensuring the outlet valve opens normally to relieve pressure.
Real-World Cases
BYD DTC AI Analysis

Worn ABS pump wiring harness causing intermittent C001904 fault on BYD E5

✓
A 2018 BYD E5 showed an intermittent ABS warning light. DTC C001904 (left rear outlet valve fault) read; status intermittent. Checked ABS pump wiring harness. Found insulation worn through near the firewall retaining clip in the engine bay due to long-term vibration. Green/white wire (left rear outlet valve drive) intermittently shorting to earth. Repaired harness and secured routing. Fault resolved.
BYD DTC AI Analysis

BYD E5 ABS hydraulic modulator assembly internal solenoid valve open circuit

✓
A 2019 E5 came in with the ABS warning light constantly on. DTC C001904 was present and would not clear. Measured infinite resistance between the left rear outlet valve pins at the ABS pump connector (normal: ~35Ω), indicating an open circuit in the hydraulic modulator's internal solenoid valve coil. Since the solenoid valve and hydraulic modulator are integrated, replacing the valve body alone is uneconomical and risks seal failure. Replaced the complete ABS hydraulic modulator assembly (part number: 3530100-B...). Performed the bleed procedure and sensor calibration; fault resolved.
BYD DTC AI Analysis

Connector corrosion after water wading caused left rear outlet valve circuit fault.

✓
After driving through flood water, the ABS warning light illuminated. The diagnostic scanner retrieved code C001904. Removed the ABS pump wiring harness connector and found significant water ingress and verdigris corrosion inside, causing excessive contact resistance at the left rear outlet valve control pin (measured 5.8 ohms, exceeding the <1 ohm standard). Cleaned the connector pins, sprayed electronic contact cleaner, applied insulating silicone grease, and checked the waterproof seal integrity. Cleared the fault codes and road tested; ABS function returned to normal.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.