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) — Qin Plus

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.

The C001904 fault code on the Qin Plus indicates an electrical fault in the left rear ABS outlet solenoid valve circuit inside the hydraulic control unit. Severity is medium because ABS/ESP may be unable to reduce brake pressure at that wheel, affecting stability during hard braking. Common causes include an open or shorted solenoid circuit or poor connector contact from corrosion or loose pins. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
3
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 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 pins backed out, oxidized, corroded, or making poor contact, interrupting the drive signal.
  • 3ABS ECU internal solenoid valve drive circuit damaged (e.g., MOSFET power transistor breakdown or driver chip fault), failing to supply approximately 5-6A 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 typically form an integrated unit; internal ribbon cable fault)
  • 5In extreme cases, severe brake fluid contamination causes the valve spool to stick, 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. Measure the left rear outlet solenoid valve resistance using a multimeter (refer to the specific vehicle repair manual for pin assignments, typically MR or ML+ and the corresponding Out terminal). The normal value is 20-60Ω. If the resistance is infinite or deviates significantly from the standard value, the internal solenoid valve of the hydraulic modulator is faulty.
  • 2
    Step 2: Inspect 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 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). If the drive signal is absent, 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 the 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 road surface (such as wet asphalt). Confirm the ABS activates normally with no 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 Qin Plus 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.