AU
C100104

DTC C100104 has dual definitions across different BYD vehicle platforms — Seal U

Braking System

DTC C100104 has dual definitions across different BYD vehicle platforms.

On early pure electric platforms like the E5, this code explicitly indicates an internal CAN hardware communication fault within the ABS control unit.

This indicates an abnormality at the physical or data link layer between the ABS module and the vehicle CAN network (powertrain CAN or chassis CAN).

The fault can interrupt communication or cause data packet loss in brake-related systems like the ABS, ESP, and EPB.

On DM-i and e-Platform 3.0 models like the Song Pro, Tang DM, and Qin Plus, this code typically maps to an abnormal left front wheel speed sensor (WSS) signal or circuit fault.

Regardless of the platform, this is a safety-critical braking system fault.

The fault degrades or completely disables ABS/ESC functions.

The instrument cluster typically illuminates the ABS warning lamp, ESC warning lamp, and parking system fault lamp.

CAN hardware faults typically originate from a damaged internal ABS module transceiver, abnormal terminating resistance, or wiring physical layer faults.

Wheel speed sensor faults mostly originate from sensor component failure, wiring harness open or short circuits, or an abnormal signal tone ring.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Hardware damage to the internal CAN transceiver in the ABS control unit or communication chip failure (early models such as E5)
  • 2Short circuit, open circuit, or abnormal resistance in the left front wheel speed sensor internal coil (standard value: 1.0-1.6kΩ)
  • 3Worn or broken wheel speed sensor wiring harness insulation, or excessive contact resistance caused by water ingress and oxidation in the connector.
  • 4Deformed sensor mounting bracket causes an abnormal gap between the sensor and the driveshaft magnetic tone ring (outside the standard range of 0.3-1.2 mm).
  • 5Left front halfshaft reluctor ring deformed, missing teeth, contaminated, or has degraded magnetism.
What To Do
  • 1
    Use VDS2000 or Launch X431 to read the complete DTC stream. Confirm whether C100104 is a current or historical fault. Check for accompanying U-class communication DTCs (such as CAN bus off) or other wheel speed sensor DTCs to initially determine the fault type (CAN hardware or wheel speed signal).
  • 2
    For CAN hardware faults on E5 and similar models: Check the ABS module power supply (constant +BAT, ignition IG1), ground (GND), and CAN-H/CAN-L circuit voltages (standard: CAN-H 2.5-3.5V, CAN-L 1.5-2.5V, static differential approx. 2.0V). Measure the chassis CAN termination resistance at the OBD diagnostic port (approx. 60Ω).
  • 3
    For a wheel speed sensor fault: Raise the vehicle, disconnect the left front wheel speed sensor connector, and measure the sensor resistance using a multimeter (1.0-1.6 kΩ at 20°C). Inspect the inside of the connector for water stains, oxidation, or recessed pins.
  • 4
    Inspect the left front wheel speed sensor wiring harness routing. Focus on the wheel arch liner, steering knuckle retaining clips, and suspension movement areas. Check for harness interference or chafing against the tire sidewall or suspension components. Measure harness continuity and insulation to ground/power (must be greater than 10MΩ).
  • 5
    Remove the sensor and inspect the mounting bracket for deformation. Use a feeler gauge to measure the gap between the sensor end face and the half-shaft tone ring (standard: 0.3-1.2 mm). Inspect the tone ring for impact deformation, cracks, or attached iron filings. Clean the tone ring or replace the half-shaft assembly if necessary.
  • 6
    Based on the diagnostic results, replace the ABS control module (CAN hardware fault) or the wheel speed sensor/wiring harness/half shaft. During installation, tighten the sensor to the specified torque (usually 8-10 N·m). Clear the fault codes and road test the vehicle to confirm normal ABS/ESC operation.
Real-World Cases
BYD DTC AI Analysis

Song Pro DM sensor body short circuit

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Symptoms: After starting the vehicle, the ABS, ESP and EPB warning lights on the dash illuminate simultaneously and stay on constantly. The speedometer needle occasionally drops to zero, and the ESP system falsely triggers automatic braking. DTC C100104 shows as a current fault and will not clear. Diagnosis: Connected Launch X431 and retrieved chassis system DTC C100104 (Left Front Wheel Speed Sensor Signal Abnormal). Raised the vehicle and disconnected the left front wheel speed sensor connector. Measured sensor resistance at 0.2Ω (specification: 1.0-1.6kΩ), confirming an internal coil short circuit. Inspection revealed visible impact damage to the sensor head from a previous underbody scrape. Solution: Replaced the left front wheel speed sensor (Genuine Part No. 3635100-). Cleared the DTCs and road tested. Live data confirmed normal left front wheel speed signal.
Original source ↗
BYD DTC AI Analysis

Tang DM sensor connector water ingress and oxidation

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Symptoms: After heavy rain or driving through water, the dash displays 'Check ESP System' and '4WD System Limited'. The 4WD mode automatically exits, DTC C100104 sets intermittently, and the code clears automatically after driving in dry weather. Diagnosis: Scanned with VDS2000 and found C100104: 3 history faults, 1 current fault. Inspected the left front wheel speed sensor connector (near the left front wheel arch liner). Found visible water inside the connector and green oxidation on the metal pins. Measured sensor resistance at 1.3kΩ (normal); wiring continuity checked good. High contact resistance at the connector caused signal distortion. Fix: Cleaned the connector pins thoroughly with precision electronic contact cleaner, sanded off the oxidation with fine sandpaper, and applied dielectric grease to prevent further oxidation. Wrapped the connector with waterproof insulation tape and rerouted the wiring harness to prevent water pooling.
Original source ↗
BYD DTC AI Analysis

Qin Plus EV wiring harness wear causing intermittent open circuit

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Symptoms: ABS warning light illuminates intermittently on rough roads or speed bumps, clears automatically on smooth surfaces. Fault correlates with body vibration. DTC C100104 status: intermittent. Diagnosis: Inspected left front wheel speed sensor harness routing. Found the retaining clip at the knuckle had detached; the harness drooped and chafed against the tire sidewall. Insulation wore through, two-thirds of internal copper strands fractured, leaving only a few intact. At-rest continuity read normal, but resistance went infinite when flexing the harness. Repair: Cut out the damaged harness section (approximately 15 cm). Extended the wiring using soldered joints—do not use simple twist connections to prevent signal interference. Sealed with double-layer heat shrink tubing. Re-secured the harness with cable ties, maintaining 20 mm clearance from the tire and suspension moving parts. Installed a rubber sleeve at the mounting point.
Original source ↗
BYD DTC AI Analysis

Han DM: Improper repair caused abnormal sensor clearance

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Symptoms: After the front brake pads were replaced at a shop, the ABS warning light stayed on, auto hold failed, and cruise control wouldn't engage. Fault codes C100104 and C003708 (left front wheel speed sensor signal implausible) appeared simultaneously. Diagnosis: The left front wheel speed sensor mounting bracket had obvious pry marks (the technician damaged it while prying off the brake caliper). A feeler gauge measured the gap between the sensor and reluctor ring at 2.8 mm, far exceeding the 0.3–1.2 mm specification. The excessive gap weakened the magnetic induction signal, so the ECU couldn't recognize a valid wheel speed signal. Resolution: Removed the sensor and straightened the bent bracket using hand tools. Reinstalled the sensor with the gap set to the standard 0.8 mm, ensuring the sensor face remained parallel to the reluctor ring. Torqued the mounting bolt to 8 N·m to avoid deforming the bracket. Cleared all fault codes and road-tested to confirm.
Original source ↗
BYD DTC AI Analysis

Yuan Plus half-shaft tone ring deformed

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Symptoms: Low-speed driving (10-20 km/h) triggers frequent ABS pump cycling with a thumping noise and brake pedal kickback. Above 40 km/h the symptom disappears, but DTC C100104 remains. The left front wheel had previously hit the kerb. Diagnosis: Live data showed abnormal fluctuation in the left front wheel speed signal at low speeds, with occasional spikes to 255 km/h (signal overflow). Removed and inspected the left front half shaft; the magnetic tone ring (integrated on the outer CV joint) had local deformation with metal debris attached. The deformation caused periodic air-gap variations between the ring and sensor, generating an incorrect signal waveform. Solution: Cleaned the tone ring surface. Attempted to straighten the deformation but achieved poor results (the tone ring is a single-piece stamping that cannot be accurately repaired). Replaced the left front half shaft assembly (including the tone ring). Preventatively replaced the rear wheel speed sensors (as the impact may have caused internal damage). Fault completely resolved.
Original source ↗
Other Seal U 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.