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 — Seal 6 EV

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.

The C003804 fault code on the Seal 6 EV indicates a medium-severity issue with the left rear wheel speed sensor circuit. The ABS/ESC module is not receiving a valid speed signal, which may limit ABS, ESC, AEB, and traction control. Common causes include damaged sensor wiring shorting to the body or corroded/loose connector pins. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Left rear wheel speed sensor wiring harness insulation worn, shorting to ground against sharp edges of the frame rail, suspension control arm, or seat bracket (common after driving on rough roads).
  • 2Sensor connector (K08A) pins loose, backed out, oxidized due to water ingress, or increased contact resistance, interrupting signal transmission.
  • 3Left rear wheel speed sensor internal coil open or short circuit (displays as a wiring fault code, but is actually an internal sensor circuit fault)
  • 4Poor solder joints on the internal circuit board of the instrument panel power distribution box (such as G2E) or burnt relay contacts disrupt sensor power supply or signal transmission.
  • 5Body manufacturing burr piercing the wiring harness, or a detached harness retaining clip causing 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 verify C003804 is an active fault. View the live data stream to confirm the left rear wheel speed displays 0 km/h or a fixed abnormal value (such as 2252 km/h) and differs significantly from the other wheel speeds.
  • 2
    Raise the vehicle. Visually inspect the left rear wheel speed sensor installation. Clean metal filings from the sensor head. Measure the gap between the sensor and the tone ring (standard: 0.3-1.2mm). 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 between the power pin (pin 1, usually black/red wire) and ground; it should be 12V ± 0.5V. Measure the resistance between the ground pin (pin 2, usually brown/yellow wire) and ground; it should be less than 1Ω.
  • 4
    Measure signal circuit continuity: resistance from the sensor connector to the corresponding pin on 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
    Thoroughly inspect the left rear wheel wiring harness routing (via the left rear door sill, under the seat, and the C-pillar). Check the harness fixing points and body panel pass-through holes for abrasion, insulation damage, or crushing marks. Check if body burrs under the seat have pierced the wiring harness.
  • 6
    Check the instrument panel power distribution box (e.g., K2G connector) and the main body floor wiring harness connector. Verify no water ingress, oxidation, or pin back-out. If necessary, measure voltage stability at the power distribution box output terminals.
  • 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 lamp 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 Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.