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 U

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 — Seal U
  • 1Worn left rear wheel speed sensor wiring harness insulation shorts to ground against sharp edges of the body rail, suspension control arm, or seat bracket (common after driving on rough roads).
  • 2Sensor connector (K08A) pins are loose, backed out, oxidized due to water ingress, or have increased contact resistance, interrupting signal transmission.
  • 3Open or short circuit in the left rear wheel speed sensor internal coil (although indicated as a wiring fault code, the actual fault is within the sensor's internal circuit).
  • 4Cold solder joints on the internal circuit board of the dashboard power distribution box (e.g., G2E) or burnt relay contacts causing abnormal sensor power supply or signal transmission.
  • 5Body manufacturing burrs pierced the wiring harness, or a detached wiring harness retaining clip caused dynamic interference between the wiring harness and rear wheel moving parts.
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). View the live data stream to verify the left rear wheel speed displays 0 km/h or a fixed abnormal value (such as 2252 km/h), differing 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: 0.3-1.2 mm). Check 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 supply 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: The resistance between the sensor connector and the corresponding pin of the ABS control unit (or ESP module) must be less than 1 Ω. Measure the 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 routing (via the left rear door sill, under the seat, and C-pillar). Check the harness fixing points and body panel 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 (such as the K2G connector) and the large body floor wiring harness connector to confirm no water ingress, oxidation, or terminal back-out. 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). During installation, apply an appropriate amount of grease and fully insert the sensor into position.
  • 8
    Clear the fault code and perform a road test including straight-line acceleration, turning, and braking. Verify the left rear wheel speed in the data stream matches the other wheel speeds and the ABS/ESC warning lamp remains off.
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 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.