AU
C001004

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

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.

The C001004 fault code on the Seal U indicates a medium-severity fault with the left front ABS inlet valve or its control circuit. This can reduce ABS function at that wheel and limit ESC, although normal braking usually remains. Common causes include a failed solenoid coil in the ABS hydraulic modulator or corroded/loose connector pins from water ingress. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Open circuit or inter-turn short circuit in the internal left front inlet valve solenoid coil of the ABS hydraulic modulator assembly (prolonged high-temperature operation causes insulation aging)
  • 2Oxidation, backed-out pins, or corrosion from water ingress 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 provide the approximately 5A peak holding current required by the valve body.
  • 4Overdue brake fluid replacement (over 2 years or 40,000 km) causes impurity buildup that sticks the solenoid valve spool in the open or closed position, resulting in 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 wires while 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 C001000 (drive fault) or C001100 (outlet valve fault) codes. 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 the E5 model year). Check the terminals for green copper corrosion, backed-out pins, or signs of burning. Clean with electrical contact cleaner and apply conductive grease.
  • 3
    Measure the left front inlet valve solenoid resistance: 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). Standard value: 3.0-6.0 Ω at 20°C. If the resistance is <1 Ω (short circuit) or >10 Ω (open circuit), the hydraulic unit internal valve body is faulty.
  • 4
    Perform valve body actuation test: Use the diagnostic tool to execute the 'ABS solenoid valve actuation test' function. Listen for a clear 'click' from the left front inlet valve. If no sound occurs 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 the self-test. A normal reading shows a 12V square-wave pulse. If the voltage remains at 0V or a constant 12V, the ECU internal driver stage is damaged.
  • 6
    Repair strategy: For the E5 model, the Bosch 9.0 or 9.3 generation ABS unit features a non-serviceable design. If diagnostics confirm an internal fault in the valve body or ECU, replace the complete ABS hydraulic modulator assembly (with ECU) and perform coding matching and ESC sensor calibration.
  • 7
    Post-repair verification: After replacement, perform the bleeding procedure (trigger the hydraulic pump cycle using the diagnostic tool). Conduct 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 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.