AU
C003804

This DTC indicates the ABS control unit (ESC module) detected abnormal electrical characteristics in the left rear wheel speed sensor circuit, including a signal line open circuit, short to ground, short to power, or intermittent connection — Atto 8

Braking System

This DTC indicates the ABS control unit (ESC module) detected abnormal electrical characteristics in the left rear wheel speed sensor circuit, including a signal line open circuit, short to ground, short to power, or intermittent connection.

The ABS ECU fails to receive the square-wave speed signal from the left rear wheel, or the signal voltage falls outside the threshold range (normal static voltage approximately 11V; dynamic switching 0.5-11V).

This fault triggers the ABS fail-safe mode, disabling or limiting the anti-lock braking, electronic stability control (ESC), autonomous emergency braking (AEB), and traction control (TCS) functions, and illuminates the ABS/ESC warning lamp on the instrument cluster.

Due to the compact chassis layout and long wiring harness on pure electric models like the BYD E5, vehicle vibration rubbing the harness against sharp body edges, connector oxidation after water exposure, or poor internal contact in the power distribution box frequently cause this fault.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Worn left rear wheel speed sensor wiring harness insulation causing a short to ground against sharp edges of the body frame, suspension control arm, or seat bracket (common after driving on rough roads).
  • 2Loose or backed-out pins, oxidation due to water ingress, or increased contact resistance at sensor connector (K08A), interrupting signal transmission.
  • 3Open or short circuit in the left rear wheel speed sensor internal coil (displays as a wiring fault code, but actually indicates an internal sensor circuit fault)
  • 4Cold solder joints on the internal circuit board of the instrument panel power distribution box (e.g., G2E) or burnt relay contacts causing abnormal sensor power supply or signal transmission.
  • 5Body manufacturing burrs pierce the wiring harness, or a detached wiring harness retaining clip causes dynamic interference between the wiring harness and rear wheel moving components.
What To Do
  • 1
    Use the BYD VDS or a dedicated diagnostic tool to read the ABS system fault codes and confirm C003804 is a current fault (Active). Check the live data stream and confirm the left rear wheel speed displays 0 km/h or a fixed abnormal value (such as 2252 km/h) that differs significantly from the other wheel speeds.
  • 2
    Raise the vehicle and visually inspect the left rear wheel speed sensor installation. Clean metal filings from the sensor tip, measure the gap between the sensor and the tone ring (standard 0.3-1.2mm), and inspect the tone ring for missing teeth or contamination.
  • 3
    Disconnect the left rear wheel speed sensor connector (K08A). Turn the ignition switch ON. Measure the voltage from the power supply pin (pin 1, usually black/red wire) to ground; it must be 12V ± 0.5V. Measure the resistance from the ground pin (pin 2, usually brown/yellow wire) to ground; it must be less than 1Ω.
  • 4
    Measure signal circuit continuity: Resistance from the sensor connector to the corresponding pin of the ABS control unit (or ESP module) must be less than 1 Ω. Measure insulation resistance between the circuit and the vehicle body; it must be greater than 10 MΩ. Continuity to ground indicates a short circuit.
  • 5
    Inspect the left rear wheel wiring harness along its route (via the left rear door sill, under the seat, and the C-pillar). Check the harness fixing points and body panel holes for wear, insulation damage, or crushing marks. Check if body burrs under the seat have pierced the wiring harness.
  • 6
    Check the dashboard power distribution box (such as the K2G connector) and the large body floor wiring harness plug. Verify no water ingress, oxidation, or backed-out pins. If necessary, measure the voltage stability at the power distribution box output terminal.
  • 7
    If all circuit measurements are normal, replace the left rear wheel speed sensor (on some models, replace it as an assembly with the wheel bearing unit). Apply an appropriate amount of grease during installation and fully insert the sensor into position.
  • 8
    Clear the fault code and perform a road test (including straight-line acceleration, turning, and braking conditions). Verify the left rear wheel speed in the data stream matches the other wheel speeds and the ABS/ESC warning light no longer illuminates.
Real-World Cases
BYD DTC AI Analysis

BYD F3 Sedan ABS Wiring Harness Chafed and Shorted to Ground

✓
A BYD F3 with approximately 103,000 km developed an intermittent ABS warning light while driving. Live data showed an abnormal right rear wheel speed of 2252 km/h, though the actual fault was left rear wheel speed sensor circuit C003804. Replacing the sensor did not help. The wiring harness had continuity to earth. Inspection found a white/red wire at the bottom of the right chassis rail had chafed through and grounded against the body. Wrapping the damaged section and securing the harness to specification resolved the fault.
Original source ↗
BYD DTC AI Analysis

BYD Qin Pro DM body burr pierced wiring harness

✓
BYD Qin Pro DM ESP warning light on, DTCs C003804 and U100308 stored. Replaced left and right wheel speed sensors; fault returned. Pin K08A-1 had no power, while K08B-1 had 12 V. Found a manufacturing burr under the left rear seat at the body ground point had pierced the harness, causing an intermittent short. Insulated the burr and repaired the harness. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Internal damage to BYD Song MAX instrument panel power distribution box

✓
The BYD Song MAX instrument cluster displayed "Check ESP System" and "Check ABS System" warnings with DTCs C003708 and C003804. Replaced the left rear wheel speed sensor and test drove 5 km; the fault returned. Voltage at sensor pin 2 measured only 2.789V (normal: 11V). Wiggling the K2G connector at the instrument panel distribution box caused voltage fluctuations. Internal circuit damage in the instrument panel distribution box was confirmed. Replaced the distribution box and cleared the fault.
Original source ↗
BYD DTC AI Analysis

BYD F3 Wheel Speed Sensor Wiring Harness Troubleshooting

✓
The BYD F3 ABS warning light stayed on. Retrieved DTC C003804. Performed systematic diagnosis per the workshop manual, measuring the left rear wheel speed sensor supply, ground, and signal circuits. Found an internal open circuit in the sensor, plus minor oxidation from water ingress in the wiring harness connector. Cleaned the connector terminals and replaced the wheel speed sensor. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD E5: Intermittent wheel speed sensor signal loss

✓
A 2018 BYD E5 came in with the ABS light flickering intermittently while driving. The scan tool retrieved historic DTC C003804. Inspection found the left rear wheel speed sensor wiring harness retaining clip had detached where it passes through the left rear wheel arch liner, allowing the harness to chafe against the liner and causing intermittent signal loss over bumps. Re-secured the harness and wrapped the worn section. The fault has not returned.
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.