AU
C001004

DTC C001004 indicates the ABS electronic control unit (ECU) detected an electrical or functional fault in the left front wheel inlet valve — Atto 8

Braking System

DTC C001004 indicates the ABS electronic control unit (ECU) detected an electrical or functional fault in the left front wheel inlet valve.

The ECU controls this key solenoid valve within the ABS hydraulic modulator assembly via an H-bridge driver circuit to regulate brake fluid flow from the master cylinder to the left front wheel cylinder during ABS/ESC intervention.

Fault trigger mechanisms include a solenoid valve coil open or short circuit (abnormal resistance), a driver circuit short to power or ground, mechanical valve sticking causing abnormal feedback current, or internal ECU MOSFET driver stage damage.

This fault disables the left front wheel ABS anti-lock function and limits the ESC stability control system, but the vehicle usually retains normal hydraulic braking.

Because the E5 uses an integrated ABS/ESC hydraulic unit (HEU) combining the valve body and ECU into a single assembly, the fault may involve either the hydraulic mechanical section or the electronic control section.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Open circuit or inter-turn short circuit in the internal left front inlet valve solenoid coil of the ABS hydraulic modulator assembly (insulation aging from prolonged high-temperature operation)
  • 2Oxidation, terminal back-out, or water ingress corrosion in the internal wiring harness or external connector pins between the ABS hydraulic modulator and the ECU, interrupting signal transmission.
  • 3ABS ECU internal H-bridge drive circuit fault (power MOSFET breakdown or pre-driver chip damage), unable to supply the valve body with the required approximately 5A peak holding current.
  • 4Failure to replace brake fluid (over 2 years or 40,000 km) leads to impurity buildup, causing the solenoid valve spool to stick in the open or closed position and generating abnormal current feedback.
  • 5After a front-end collision, the deformed left front longitudinal beam stretched the ABS hydraulic unit wiring harness, breaking the internal copper strands but leaving the insulation intact (hidden open circuit).
What To Do
  • 1
    Connect the VDS or ED-400 diagnostic tool. Read the complete fault codes and record the Freeze Frame data. Check for accompanying codes C001000 (drive fault) or C001100 (oil outlet valve fault). Clear the fault codes and perform a road test to reproduce the fault.
  • 2
    Disconnect the electrical connector from the ABS hydraulic modulator assembly (24-pin or 28-pin, depending on E5 model year). Inspect the pins for green corrosion, backed-out pins, or burn marks. Clean with electrical contact cleaner and apply conductive grease.
  • 3
    Measure the resistance of the left front inlet valve solenoid: Use a multimeter to measure the corresponding pins on the hydraulic unit connector (typically the INLET FL valve drive terminal defined by the ECU pinout). The standard value is 3.0-6.0 Ω (at 20°C). If the resistance is <1 Ω (short circuit) or >10 Ω (open circuit), the internal valve body of the hydraulic unit is faulty.
  • 4
    Perform valve body operation test: Use the diagnostic tool to execute the 'ABS solenoid valve actuation test' function and listen for a crisp click from the left front inlet valve. If the valve makes no sound but resistance is normal, this indicates mechanical binding or blocked internal hydraulic passages in the hydraulic unit.
  • 5
    Check ECU drive capability: If valve body resistance and mechanical operation are normal, check the ECU-side drive circuit. Use an oscilloscope to measure the voltage waveform at the ECU output during self-check. The oscilloscope should display a 12V square wave pulse. A constant 0V or 12V indicates a damaged ECU internal driver stage.
  • 6
    Repair strategy: For E5 models, Bosch generation 9.0 or 9.3 ABS units use a non-repairable design. If diagnostics confirm an internal fault in the valve body or ECU, replace the complete ABS hydraulic modulator assembly (with ECU), then perform coding and ESC sensor calibration.
  • 7
    Post-repair verification: After replacement, perform the bleeding procedure (use a diagnostic tool to trigger the hydraulic pump cycle). Perform emergency braking tests at 30 km/h and 50 km/h to confirm the ABS activates normally and the fault code does not recur.
Real-World Cases
BYD DTC AI Analysis

E5 commercial vehicle: ABS pump assembly internal valve body open circuit

✓
2018 BYD E5 ride-share vehicle, 120,000 km. ABS and ESC warning lights on simultaneously. Scanner read codes C001004 and C001100 (left front outlet valve fault). Pulled the ABS hydraulic unit connector—no visible issues. Resistance measured infinite (open circuit) at the left front inlet valve; the outlet valve tested normal. The solenoid coil inside the hydraulic unit’s left front channel had burnt out. The E5 ABS unit is integrated—you cannot replace the valve body separately. Replaced the complete ABS hydraulic modulator assembly (P/N: BYD-3631010-C1), bled the system, and calibrated the lateral acceleration sensor. Fixed.
BYD DTC AI Analysis

Wiring harness short circuit after accident repair caused valve body drive fault

✓
A 2019 E5 sustained left-front collision damage. After a workshop replaced the left front chassis rail, the ABS warning light illuminated. Scanning retrieved only DTC C001004. Inspection found that during the accident repair, the left front wheel speed sensor harness and ABS hydraulic unit harness had been secured too tightly together, causing a metal bracket to chafe through the insulation and short the inlet valve drive wire to ground. Repaired the damaged harness by re-soldering the wires and insulating with heat-shrink tubing. Routed the harnesses separately to prevent interference. Cleared the fault.
BYD DTC AI Analysis

Contaminated brake fluid caused the solenoid valve to stick intermittently

✓
2018 E5. Owner reported the ABS warning light coming on intermittently, especially in rain or after washing the vehicle. Scan tool showed historic DTC C001004 with intermittent status. Found brake fluid severely blackened with moisture content over 3%. Disassembled the ABS hydraulic unit inlet filter and found substantial flocculent debris. Ran the ABS hydraulic unit cleaning procedure (circulated cleaning fluid through the valve body using specialized equipment), replaced DOT4 brake fluid, and thoroughly bled the system. Fault has not returned. Advised owner to replace brake fluid every 2 years.
BYD DTC AI Analysis

Loose connector pins caused an intermittent connection fault

✓
An E5 with 80,000 km showed occasional ABS warning light flicker on rough roads. Scanned with a diagnostic tool, DTC C001004 appeared during vehicle vibration. Inspection of the ABS hydraulic unit 24-pin connector found pin 15 (left front inlet valve drive) slightly backed out with a loose socket. Removed the terminal with a special extraction tool, reamed the opening, re-crimped it, applied electrical contact grease, and fully locked the connector latch. Road testing confirmed the fault resolved.
Other Atto 8 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.