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 8

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 Atto 8 indicates an electrical fault in the left rear ABS/ESP outlet solenoid valve circuit inside the hydraulic control unit. Severity is medium because ABS or stability control may not reduce brake pressure correctly at that wheel. Common causes include an open or shorted solenoid coil, or poor contact at the ABS pump wiring connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from 3 recorded cases.
3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 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) pins backed out, oxidized, corroded, or making poor contact, 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 5-6A drive current.
  • 4Open or short circuit in the internal wiring harness or mating connector between the ECU and the 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 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 valve solenoid resistance using a multimeter (refer to the specific vehicle repair manual for pin positions, usually MR or ML+ corresponding to the Out terminal). The standard resistance is 20-60 Ω. Infinite resistance or a value significantly outside the standard range indicates a faulty internal solenoid valve in the hydraulic modulator.
  • 2
    Step 2: Wiring harness continuity and insulation check. In the disconnected state, 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 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 the drive signal is absent, diagnose an internal ABS ECU fault.
  • 4
    Step 4: Assembly replacement and adaptation. If diagnostics confirm 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 dedicated 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 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, confirming the outlet valve opens properly 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 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.