AU
C001104

DTC C001104 indicates a circuit or functional fault in the left front wheel outlet valve (LF Outlet Valve) inside the ABS (Anti-lock Braking System) Hydraulic Electronic Control Unit (HECU) assembly — Atto 3

Braking System

DTC C001104 indicates a circuit or functional fault in the left front wheel outlet valve (LF Outlet Valve) inside the ABS (Anti-lock Braking System) Hydraulic Electronic Control Unit (HECU) assembly.

This solenoid valve integrates into the ABS pump body.

During the ABS pressure reduction phase, it returns high-pressure brake fluid from the left front brake caliper to the reservoir or master cylinder, rapidly reducing wheel cylinder pressure and preventing wheel lock-up.

Fault trigger conditions include an open or short circuit in the solenoid coil, a mechanically stuck valve core, a damaged internal ABS ECU driver circuit (MOSFET), or an open or short circuit in the control wiring.

This fault disables the left front wheel ABS pressure reduction function.

During emergency braking, this wheel may lock up prematurely, causing the vehicle to pull or lose directional control and severely compromising driving safety.

The C001104 fault code on the Atto 3 indicates a medium-severity fault with the left front ABS outlet valve inside the HECU assembly. This can stop ABS pressure reduction at that wheel, increasing lock-up risk during hard braking. Common causes include a failed or stuck solenoid valve, or loose, corroded, or damaged ABS pump connector wiring. 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 or short circuit in the left front outlet valve solenoid coil inside the ABS hydraulic modulator assembly, or stuck valve spool (internal mechanical fault)
  • 2Oxidized, loose, or backed-out ABS pump connector pins, or poor wiring harness connection, interrupting the solenoid valve power supply (pin 25) or control signal.
  • 3ABS ECU internal solenoid valve drive circuit damaged (e.g., burnt driver chip), failing to output approximately 12 V drive current to the solenoid valve.
  • 4Solenoid valve power supply circuit fault, including blown 25A ABS fuse in the engine compartment, poor contact at the fuse holder, or excessive resistance in the ground circuit (pin 38).
  • 5Interference or short circuit in related wiring (e.g., damaged left front wheel speed sensor harness shorted to ground), causing the ECU to incorrectly determine the solenoid valve circuit status.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS2000/VDS2100) to read the fault code, verify C001104 is present, and check the freeze frame data to confirm the vehicle speed and system status when the fault occurred.
  • 2
    Perform the ABS system 'Component Actuation Test', select the left front outlet valve (LF Outlet Valve), and listen for a clear 'click' sound to determine if the solenoid valve actuates normally.
  • 3
    Perform a visual inspection: Check the ABS hydraulic modulator connector (usually a 25-pin or 38-pin black plug) for looseness. Inspect pin 25 (solenoid valve power supply) and pin 38 (ground) for oxidation, burning, or brake fluid contamination.
  • 4
    Measure the solenoid valve supply voltage: Set the ignition switch to ON. Use a multimeter to measure the voltage between connector pin 25 and ground. The standard value is 10-16V. Also check the 25A ABS fuse and base in the engine compartment fuse box for burning or deformation.
  • 5
    Measure solenoid valve coil resistance: Disconnect the ABS pump body connector and use a multimeter to measure the resistance between the left front outlet valve drive terminals. The normal value is typically 5-10 Ω (refer to the vehicle repair manual for exact specifications). Infinite resistance indicates an open circuit in the coil; 0 Ω indicates a short circuit in the coil.
  • 6
    Perform a circuit continuity check: Use a multimeter to measure continuity of the left front outlet valve control wire between the ABS ECU connector and the pump body connector to rule out an open circuit or high resistance. Use an oscilloscope or breakout box to monitor voltage stability while driving to rule out intermittent poor contact.
  • 7
    If wiring and resistance are normal but the component test fails, or if the fault transfers after swapping the ABS pump with one from a known good vehicle, this indicates an internal ABS pump fault (solenoid valve or ECU drive circuit). Replace the ABS hydraulic modulator assembly with the ECU (Note: do not replace the solenoid valve separately).
  • 8
    After replacement, perform the brake system bleeding procedure (use the diagnostic tool to execute 'Bleeding Mode' to activate the oil pump). Clear the fault codes and perform a road test (maintain a vehicle speed above 30 km/h for more than 10 seconds to trigger the ABS self-check). Confirm the system operates normally and the fault light is off.
Real-World Cases
BYD DTC AI Analysis

BYD Qin - ABS Pump Internal Solenoid Valve Fault

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Symptoms: Dashboard ABS and brake system warning lights stay on. Scanner shows DTC C001104. ABS inoperative while driving but conventional braking works normally. Diagnostic process: Performed actuator test — left front outlet valve shows no response. Measured 12.4V between connector pins 25 and 38 (normal). 25A fuse checks good. Swapped ABS pump with unit from a known-good vehicle; the fault code transferred with the pump, confirming internal ABS pump solenoid valve failure. Solution: Replaced ABS hydraulic modulator assembly with ECU. Cleared codes and road tested (straight-line driving above 30 km/h for at least 10 seconds). Fault resolved; ABS warning light off.
Original source ↗
BYD DTC AI Analysis

BYD Song DM — Poor contact in ABS solenoid valve wiring

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Symptoms: ABS and brake system warning lights come on after starting. DTC C001104 sets intermittently and returns after clearing following a period of driving. The lights come on more easily on rough roads. Diagnosis: Found slight oxidation on pin 25 (solenoid valve power terminal) of the ABS hydraulic modulator connector; solenoid coil resistance measured normal; monitored pin voltage with a breakout box while driving and caught instantaneous drops over bumps (from 12V to below 8V); traced the harness and found a loose connection at the root of the ABS pump body connector. Solution: Cleaned and treated the oxidized pins on the ABS hydraulic modulator connector, re-secured and wrapped the wiring harness, reinforced the connector with a dedicated fixture, cleared the DTC and road-tested. Fault did not reappear.
BYD DTC AI Analysis

BYD e5 – ABS ECU control module fault

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Symptoms: Ignition switched to ON, abnormal clicking heard during ABS self-check. Instrument cluster displayed ABS fault. DTCs C001104 and C001004 (left front inlet valve fault) set simultaneously. Vehicle failed ABS self-check; system entered conventional braking mode. Diagnosis: Actuator test showed no feedback from either left front solenoid. Solenoid supply voltage measured 12V (normal), ground good. Continuity between solenoid drive terminals and ECU normal. Internal solenoid drive circuit in ABS ECU faulty; unable to drive left front solenoids. Solution: Replaced ABS hydraulic modulator assembly with integrated ECU. Bled brakes, initialized system, cleared DTCs. Performed dynamic test (checked wheel speed signals above 2 km/h). ABS function normal.
BYD DTC AI Analysis

BYD Tang - DTC triggered after brake system repair

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Symptoms: After brake fluid replacement and left front brake caliper repair, the ABS warning light came on. Fault codes: C001104 and C012104 (solenoid valve relay circuit fault). ABS was normal before this repair. Diagnosis: Checked repair history: recent brake system work. Found the ABS hydraulic modulator connector contaminated with brake fluid from installation. Measured solenoid valve supply voltage at 9.2V (below spec). The 25A fuse holder had burn marks and increased contact resistance. Brake fluid entered the connector during repair, causing the circuit fault. Resolution: Cleaned the ABS hydraulic modulator connector with electrical contact cleaner to remove brake fluid residue. Replaced the 25A fuse and fuse holder; supply voltage now above 12V. Bled the ABS system again and cleared fault codes. System returned to normal.
BYD DTC AI Analysis

BYD F3 - Wiring short circuit caused false solenoid valve fault

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Symptoms: ABS warning light came on suddenly while driving with abnormal braking feel (slight pedal vibration). DTC C001104 persisted and returned immediately after clearing. Inspection found chafing marks on the left front wheel speed sensor harness where it contacts the body. Diagnosis: Read DTC C001104 and initially suspected a left front outlet valve circuit fault. Checked ABS hydraulic modulator power and ground—both normal. While inspecting the left front wheel speed sensor circuit, found the harness insulation damaged with internal wires shorting to ground. Further inspection revealed signal interference between the wheel speed sensor circuit and solenoid valve control circuit, causing the ECU to misdiagnose a solenoid valve circuit fault; the solenoid valve itself was normal. Fix: Repaired the damaged section of the left front wheel speed sensor harness, rewrapped and insulated it, rerouted the harness to prevent chafing against the body or other components, cleared the code and confirmed the fix.
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.