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) — Seal 6 EV

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 — Seal 6 EV
  • 1Open circuit, short circuit, or abnormal resistance in the ABS hydraulic modulator assembly internal left rear outlet valve solenoid coil (normal range typically 20-60Ω, depending on specific model).
  • 2Backed-out pins, oxidation, corrosion, or poor contact at the ABS pump wiring harness connector (usually located near the engine compartment firewall), interrupting the drive signal.
  • 3Damaged ABS ECU internal solenoid valve drive circuit (e.g., MOSFET power transistor breakdown or driver chip failure), unable to provide approximately 5-6 A 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 valve spool binding, resulting in 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 inspect the pins for corrosion or backing out. Use a multimeter to measure the left rear outlet valve solenoid resistance (refer to the specific vehicle workshop manual for pin assignments, usually MR or ML+ to the corresponding Out terminal). Normal resistance is 20-60 Ω. If the resistance is infinite or significantly deviates from the standard value, the hydraulic modulator internal solenoid 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). Measure the insulation resistance of this circuit to body ground and to the power supply (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. Observe the ECU terminal for a normal 12V square-wave drive signal and current draw (approximately 4-6A). If no drive signal is present, diagnose an internal ABS ECU fault.
  • 4
    Step 4: Assembly replacement and matching. If inspection confirms damage to the hydraulic modulator internal solenoid valve or ECU drive circuit, replace the ABS hydraulic modulator assembly (with ECU). After replacement, use the BYD VDS diagnostic tool to perform: ① bleeding procedure (requires a special fluid draining tool); ② wheel speed sensor signal check; ③ lateral acceleration/yaw rate sensor calibration; ④ road test to verify ABS function.
  • 5
    Step 5: Dynamic function verification. After clearing the fault code, perform an emergency braking test at 30-40 km/h on a safe, low-adhesion surface (such as wet asphalt). 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 correctly to release 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 Seal 6 EV 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.