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C003708

C003708 is a UDS-compliant fault code; subtype '08' indicates Signal Invalid/Plausibility Failure — Qin Plus

Braking System

C003708 is a UDS-compliant fault code; subtype '08' indicates Signal Invalid/Plausibility Failure.

In the BYD E5 ABS/ESC system, this fault indicates the ABS ECU detects a persistent abnormality or intermittent interruption in the electrical signal from the left rear wheel speed sensor (RL Wheel Speed Sensor).

Specific conditions include: signal amplitude outside the normal range (too high/too low), distorted signal waveform, excessive deviation from the reference model calculation, or logical inconsistency with the other three wheel speed signals (e.g., the left rear wheel speed reads significantly lower than the others during straight-line driving).

In new energy vehicles, this fault also causes inaccurate torque calculation in the regenerative braking system (RBS), which may restrict regenerative braking functions.

When this fault triggers, the system disables ABS and ESC functions, the ABS/ESC warning lamp illuminates on the instrument cluster, and the vehicle enters conventional hydraulic braking mode.

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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Wheel speed sensor failure: Aging, open circuits, or short circuits in the internal Hall element or electromagnetic coil cause insufficient output signal amplitude or complete signal loss; metal filings accumulated on the sensor tip alter the magnetic air gap.
  • 2Abnormal sensor installation gap: The air gap between the sensor and the reluctor ring falls outside the standard range (typically 0.3-1.2 mm). A deformed, loose, or worn mounting bracket creates an excessively large or small gap, causing signal loss.
  • 3Wiring and connector faults: Repeated bending of the sensor wiring harness in suspension travel areas (such as the E5 rear suspension-to-body connection) causes internal wire breakage and insulation damage; water ingress into the connector causes oxidation, terminal back-out, or excessive contact resistance; shielding damage causes electromagnetic interference.
  • 4Tone ring (trigger wheel) defect: The tone ring integrated into the inner race of the rear wheel hub bearing is deformed, missing teeth, clogged with oil or ice/snow between the teeth, or corroded, preventing it from generating a regular magnetic reluctance variation signal.
  • 5ABS control unit internal fault: Damaged ECU internal signal acquisition circuit, A/D conversion module failure, or abnormal software calibration parameters (less common; consider only after ruling out the preceding factors).
What To Do
  • 1
    Diagnostic tool deep scan: Use VDS or a dedicated diagnostic tool to read complete fault codes and freeze frame data. Record parameters present when the fault occurred, such as vehicle speed, wheel speed values, and longitudinal acceleration. Enter live data stream mode. Observe changes in the left rear wheel speed signal while the vehicle is stationary and driving. Check for signal spikes, fixed values, or obvious desynchronization with the other three wheels.
  • 2
    Physical inspection and cleaning: Remove the left rear wheel. Check the wheel speed sensor head for metal debris, oil contamination, or physical damage. Check the rear axle tone ring for missing teeth, deformation, or corrosion. Clean the sensor head and tone ring surfaces. Check the sensor mounting bracket for looseness or deformation.
  • 3
    Sensor static measurement: Disconnect the sensor connector. Use a multimeter to measure the sensor resistance (E5 models typically measure 1.0–1.5 kΩ; refer to the workshop manual for specific values). An infinite reading indicates an open circuit; 0 Ω indicates a short circuit. Measure the insulation resistance between the sensor and body ground. The resistance must be greater than 20 MΩ.
  • 4
    Harness continuity and signal test: Disconnect the ABS pump assembly connector. Measure continuity between the sensor harness and the ABS pump (must be less than 1Ω). Check for shorts to power and ground. Reinstall the sensor and raise the vehicle. Use an oscilloscope to measure the sensor output signal waveform (rotating the wheel must produce a regular sine or square wave, with amplitude increasing as wheel speed increases).
  • 5
    Gap adjustment and verification: If the sensor is adjustable, check the gap between the sensor and the tone ring using a feeler gauge. If necessary, loosen the retaining bolt and adjust the gap to the standard value (0.3-1.2mm). For integrated sensors with an abnormal gap, check the rear axle bearing housing or replace the wheel hub unit. Clear the fault code and perform a road test (including straight-line acceleration, deceleration, and turning conditions) to confirm the fault does not reappear and normal ABS function resumes.
Real-World Cases
BYD DTC AI Analysis

Worn rear wheel speed sensor wiring harness on E5 taxi caused intermittent fault

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2018 BYD E5 taxi, 120,000 km. The instrument cluster intermittently illuminated the ABS warning light. DTC C003708 (left rear wheel speed signal fault) stored; freeze frame showed the fault occurred at 40–60 km/h. Inspection found the left rear wheel speed sensor harness had chafed at the body harness connector near the rear upper control arm. Long-term suspension movement wore the insulation, leaving internal conductors partially broken. The harness lost connection over bumps, triggering the fault. Replaced the left rear wheel speed sensor harness assembly (part number: 10238320-00), rerouted and secured the harness, and added corrugated tubing protection. Fault cleared.
BYD DTC AI Analysis

Iron filings adhered to the left rear wheel speed sensor head, causing abnormal signals.

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2019 BYD E5 private vehicle, 30,000 km. ABS warning light constant on. Scan tool shows DTC C003708. Live data: left rear wheel speed signal reads 0 at standstill, spikes momentarily on initial movement, then remains 0. Removed sensor: tip heavily coated with metal powder from brake pad wear, causing magnetic short circuit and blocking tone ring signal. Root cause: owner regularly drives near dusty construction sites without periodic brake system cleaning. Thoroughly cleaned sensor tip and tone ring surface, flushed with carburetor cleaner, reassembled. Signal returned to normal. Advised owner to check wheel speed sensor cleanliness every 20,000 km.
BYD DTC AI Analysis

Rear axle tone ring deformation distorted wheel speed signal waveform.

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An E5 with minor rear-end collision damage had the ABS warning lamp come on after repairs, storing DTC C003708. The data stream showed the left rear wheel speed signal fluctuating periodically at stable vehicle speed. Removing the left rear hub unit revealed the tone ring integrated into the wheel bearing had slight elliptical deformation from the impact—barely visible, but showing slight runout when spun. This caused the sensor-to-tone ring air gap to vary cyclically, and signal amplitude fluctuation exceeded the ECU tolerance. Fix: Replaced the left rear hub bearing unit assembly (including the tone ring). Fault cleared. After collision repairs, verify wheel speed signal waveform quality using an oscilloscope rather than just clearing fault codes.
BYD DTC AI Analysis

Loose sensor mounting bracket caused excessive clearance.

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BYD E5 2018, 80,000 km. Intermittent ABS warning light with codes C003708 and C003800 (left rear wheel speed sensor power supply fault, intermittent). Found the left rear wheel speed sensor mounting bolt loose; the sensor had backed out, gap to tone ring exceeded 2.5 mm (standard 0.3-1.2 mm). Signal readable at low speed but amplitude dropped at high speed, triggering the ECU signal fault. Sensor wobble also caused intermittent poor contact in the power line, setting the supply fault code. Fix: Torqued the sensor bolt to 8-12 Nm with threadlocker, adjusted sensor to standard gap, and fixed the power terminal contact. Both codes cleared.
Other Qin Plus 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.