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 U

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 Seal U indicates a medium-severity electrical fault in the left rear ABS outlet solenoid valve circuit inside the hydraulic control unit. It may be caused by an open or shorted solenoid coil, damaged wiring, or poor connector contact from corrosion or loose pins. Visit an authorised dealer for diagnosis and repair. Since real cases are recorded, typical repair cost data is available.
3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Open circuit, short circuit, or abnormal resistance in the internal left rear outlet valve solenoid coil of the ABS hydraulic modulator assembly (normal range usually 20-60Ω, depending on specific model)
  • 2ABS pump wiring harness connector (usually located near the engine compartment firewall): backed-out terminal pins, oxidation/corrosion, or poor contact, interrupting the drive signal.
  • 3Damaged internal solenoid valve drive circuit in the ABS ECU (such as MOSFET power transistor breakdown or driver chip fault), 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 typically an integrated assembly; internal ribbon cable fault).
  • 5In extreme cases, severe brake fluid contamination causes valve spool binding, 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 for corroded or backed-out pins. Use a multimeter to measure the left rear outlet valve solenoid resistance (refer to the specific vehicle workshop manual for pin positions, usually MR or ML+ and the corresponding Out terminal). The normal value is 20-60Ω. If the resistance is infinite or significantly deviates from the standard value, the hydraulic modulator internal solenoid valve is faulty.
  • 2
    Step 2: Inspect wiring harness continuity and insulation. With the circuit disconnected, measure the continuity resistance of the drive circuit between the ECU and the solenoid valve (should be <1Ω). Measure the insulation resistance of this circuit to body ground and to the power supply (should be >10MΩ). Repair the wiring harness if a short or open circuit exists.
  • 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 (about 4-6A). If no drive signal is present, diagnose an internal ABS ECU fault.
  • 4
    Step 4: Assembly replacement and matching. If inspection confirms a faulty internal solenoid valve in the hydraulic modulator 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 verification; 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 without abnormal noise. Simultaneously monitor the data stream to verify the left rear wheel speed signal drops normally and confirm 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 U Fault Codes
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.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.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.