AU
C001C04

DTC C001C04 indicates an electrical or mechanical fault in the right rear wheel inlet valve within the ABS/ESC Hydraulic Control Unit (HCU) — Atto 3

Braking System

DTC C001C04 indicates an electrical or mechanical fault in the right rear wheel inlet valve within the ABS/ESC Hydraulic Control Unit (HCU).

The right rear inlet valve is a normally open high-speed solenoid valve controlling the hydraulic passage from the brake master cylinder to the right rear wheel brake cylinder.

This fault indicates the ABS ECU detects an open circuit or short circuit (to power or ground) in the right rear inlet valve drive circuit, a stuck valve spool, or an abnormal response.

This prevents the ECU from accurately regulating brake pressure at the right rear wheel.

This directly affects the execution of Anti-lock Braking System (ABS), Electronic Brakeforce Distribution (EBD), Electronic Stability Control (ESC), and Automatic Emergency Braking (AEB) functions at the right rear wheel.

In extreme cases, this causes right rear wheel braking failure or lock-up, triggers multiple system warnings, and initiates fail-safe mode (conventional hydraulic braking remains operational, but electronic assist functions are limited).

The C001C04 fault code on the Atto 3 indicates a medium-severity problem with the right rear ABS/ESC inlet valve in the hydraulic control unit. It may be caused by an open or shorted solenoid circuit, abnormal valve resistance, poor connector contact, corrosion, or damaged wiring. ABS, ESC, EBD, and AEB may be limited, though basic braking should remain. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Open circuit or abnormal resistance in the right rear inlet valve solenoid coil inside the hydraulic modulator assembly (normal resistance 2-5Ω; out of tolerance).
  • 2Poor contact at the wiring harness connector between the ABS ECU and hydraulic modulator, corroded pins, or damaged wiring harness, interrupting drive signal transmission.
  • 3ABS ECU internal valve drive circuit fault (e.g., damaged MOSFET power transistor, driver chip, or current-sense resistor), unable to supply sufficient drive current (typically requires 1-2A peak current)
  • 4Severe brake fluid contamination (metal debris, moisture, or degradation) causes the inlet valve spool to stick in the open or closed position. The ECU detects this abnormality via current feedback.
  • 5Previous water wading or collision caused moisture, oxidation, or physical damage to the hydraulic modulator internal circuit board, affecting solenoid valve control.
What To Do
  • 1
    Use a dedicated diagnostic tool (such as BYD VDS or Launch X431) to read all fault codes and freeze frame data. Verify if C001C04 is a Current fault. Check for an accompanying C001C00 (right rear inlet valve drive fault) or other wheel speed sensor faults. Clear the fault codes, perform a road test, and observe if the fault returns.
  • 2
    Check the appearance of the ABS hydraulic modulator and wiring harness connector: inspect the hydraulic modulator assembly plug near the right rear wheel for looseness, water ingress, oxidation, or burn marks. Measure the wiring harness continuity between the ABS ECU and the corresponding pins of the hydraulic modulator (must be less than 1 Ω) and the insulation resistance (must be greater than 10 MΩ to prevent a short to ground or power).
  • 3
    Solenoid valve resistance measurement: Disconnect the hydraulic modulator connector. Use a multimeter to measure the right rear inlet valve coil resistance (refer to the circuit diagram for pin locations, typically the HCU pins corresponding to the RL inlet valve). Normal resistance is 2.0-5.0 Ω (refer to the E5 workshop manual for specific standard values). If the resistance is infinite (open circuit) or near 0 (short circuit), this indicates an internal fault in the hydraulic modulator assembly.
  • 4
    Actuator operation test: Connect the diagnostic tool and enter the ABS active test function. Perform the 'right rear inlet valve' actuation test. Listen with a stethoscope or touch the valve body to verify a clear 'click' switching sound. If no sound occurs but the ECU reports a drive fault, use the resistance measurement results to confirm a valve body or circuit fault.
  • 5
    Replacement and verification: If diagnostics confirm a hydraulic modulator assembly fault (the BYD E5 ABS ECU and HCU typically feature an integrated design; do not replace the solenoid valve separately), replace the complete ABS hydraulic modulator assembly. After replacement, perform: ① Brake system bleeding procedure (use a diagnostic tool to bleed the pump motor); ② Longitudinal acceleration sensor and yaw rate sensor calibration; ③ Wheel speed sensor signal check; ④ Road test to verify ABS/ESC function and DTC status.
Real-World Cases
BYD DTC AI Analysis

Replaced HCU assembly on BYD E5 taxi due to open circuit in right rear inlet valve coil

✓
A 2018 BYD E5 taxi with 280,000 km showed illuminated ABS and ESC warning lights. Scanning revealed DTCs C001C04 (right rear inlet valve fault) and C001D00 (right rear outlet valve fault). Technicians measured the hydraulic modulator connector and found infinite resistance (open circuit) between the right rear inlet valve pins; other wheel valves showed normal resistance of approximately 3.2 Ω. Disassembling the ABS assembly revealed fatigue cracks in the circuit board solder joints from long-term vibration. They replaced the ABS hydraulic modulator assembly (with ECU), clearing the fault. The assembly was out of warranty and classified as a wear item, so the owner paid for the replacement.
BYD DTC AI Analysis

Water ingress caused connector corrosion, triggering intermittent C001C04 fault.

✓
2019 E5 owner reported the ABS warning light intermittently illuminating after driving in wet conditions. DTC C001C04 stored as a historical fault. Inspection found green copper corrosion inside the 24-pin wiring harness connector for the ABS hydraulic modulator located near the right front wheel arch. The right rear inlet valve drive pin (typically pin X; refer to wiring diagram) showed slight oxidation, increasing contact resistance to 12Ω (normal: <0.5Ω). Cleaned the connector with precision electrical contact cleaner, applied conductive protective grease, and retightened. No recurrence after one month of monitoring.
BYD DTC AI Analysis

Brake fluid contamination jammed the valve spool and caused false alarms.

✓
An accident-repaired E5 developed a C001C04 fault after the workshop replaced the right rear brake caliper without thoroughly bleeding the system, then incorrectly added inferior brake fluid mixed with the original DOT4. Scanner showed the inlet valve circuit normal at 3.5Ω, but the valve made no sound during active testing. Disassembling the hydraulic modulator revealed the inlet valve spool jammed with gummy contaminants. Dedicated brake cleaning equipment failed to clear the blockage. The workshop replaced the hydraulic modulator assembly, flushed all brake lines, and replaced all brake fluid. Advised owner to replace brake fluid every 40,000 km or 2 years.
BYD DTC AI Analysis

Internal driver chip failure in the ABS ECU caused multiple valve faults.

✓
A 2018 BYD E5 triggered simultaneous DTCs C001C04 (rear right inlet valve) and C001B04 (rear left inlet valve). Resistance measured 3.0Ω and 3.1Ω at the two valves respectively, with harness continuity normal. Neither valve operated during individual actuation tests, indicating an internal ABS ECU driver circuit fault—likely a failed common ground driver chip. The BYD E5 uses an integrated ABS ECU/HCU design; repairing the ECU separately posed sealing risks. Replaced the complete ABS hydraulic modulator assembly, clearing the fault. Programmed and matched the replacement unit; vehicle returned to normal.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.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.
Common problems reported for Atto 3 →Atto 3 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.