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C1206

DTC C1206 indicates the ABS control module detected an electrical integrity fault (open or short circuit) in the left rear wheel speed sensor circuit — Seal U

Braking System

DTC C1206 indicates the ABS control module detected an electrical integrity fault (open or short circuit) in the left rear wheel speed sensor circuit.

The sensor is typically a two-wire magnetic inductive or Hall effect type.

During normal operation, it sends the ABS module either an AC voltage signal that varies with wheel speed (magnetic inductive type, amplitude increases with speed, approximately 0.1V-5V) or a square-wave digital signal (Hall effect type).

The module triggers this fault when it detects infinite resistance in the sensor wiring (open circuit), a short to ground or power, or a continuously abnormal signal voltage (below 0.5V or above 4.5V) exceeding the set threshold (typically 2-5 seconds).

This fault causes a loss of the left rear wheel speed signal, forces the ABS, EBD, ESC, TCS, and Automatic Emergency Braking (AEB) systems into a degraded mode, and limits the energy recovery system.

In extreme cases, it causes uneven brake force distribution or vehicle skidding.

4
Cases Logged
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Causes
Likely Causes — Seal U
  • 1Wheel speed sensor internal coil open or short circuit: Prolonged operation in high-temperature and high-vibration environments can fracture internal copper windings due to thermal fatigue, or damaged insulation can cause an inter-turn short circuit. This manifests as a resistance value outside the standard range (typically 1.0kΩ-2.0kΩ for BYD models).
  • 2Wiring harness and connector faults: Vehicle vibration can cause the chassis wiring harness to rub against body brackets, damaging the outer insulation and resulting in an open circuit or intermittent poor contact. Aging connector seals allow water ingress, causing pin oxidation and corrosion (especially common in wading or high-humidity areas) and interrupting signal transmission.
  • 3Incorrect sensor installation gap or damaged tone wheel (reluctor ring): Loose sensor mounting bolts cause an excessive air gap (standard gap is typically 0.3-1.0 mm). Iron filings or mud on the tone wheel teeth, tone wheel deformation, or missing teeth reduce signal amplitude, causing the system to falsely detect a circuit fault.
  • 4ABS control module internal circuit fault: A damaged sampling resistor or filter capacitor in the module's internal signal processing circuit, or a dry solder joint at the connector, prevents correct sensor signal identification and falsely reports an open/short circuit fault.
  • 5High-voltage electromagnetic interference (NEV specific): Damaged shielding or poor grounding in the high-voltage system (drive motor, DC-DC) of BYD battery electric vehicles can generate electromagnetic interference. This distorts the wheel speed sensor signal waveform and triggers a false fault code.
What To Do
  • 1
    Initial inspection and verification: Use VDS or a dedicated diagnostic tool to read the complete fault code data stream and confirm if C1206 is an active fault. Check for other wheel speed sensor or high-voltage system fault codes. Raise the vehicle and visually inspect the left rear wheel speed sensor and wiring harness for physical damage, signs of water ingress, or loose installation.
  • 2
    Sensor resistance measurement: Disconnect the left rear wheel speed sensor connector and use a multimeter to measure the resistance between the two sensor terminals. The standard value is 1.0kΩ-2.0kΩ (at 20°C). Infinite resistance indicates an open circuit, and resistance near 0Ω indicates a short circuit. If the resistance deviates significantly from the standard value, replace the sensor. Measure the insulation resistance between the sensor and body ground. The value must be greater than 10MΩ; otherwise, a short to ground exists.
  • 3
    Harness continuity and insulation test: Disconnect the ABS module connector. Measure the harness continuity resistance between the sensor connector and the corresponding pins on the ABS module (usually pins 18# and 33#; refer to the vehicle wiring diagram). The resistance must be less than 1Ω. Measure the insulation resistance of this circuit to body ground and to the 12V power supply to confirm there is no short circuit. Check the connector pins for backed-out terminals, enlarged sockets, or green corrosion.
  • 4
    Signal output verification (dynamic test): Reconnect the connector. Raise the vehicle. Use an oscilloscope or set a multimeter to AC voltage. Manually rotate the left rear wheel and observe the signal output. A magnetic inductive sensor should output a sine-wave AC voltage that increases with rotational speed (>0.2V); a Hall-effect sensor should output a 0-5V square-wave signal. If the signal is missing or weak, check the air gap between the sensor and the tone ring (standard 0.3-1.0mm). Clean foreign matter from the tone ring surface. Replace the sensor if necessary.
  • 5
    ABS module power supply and ground check: Check the ABS module 12V constant power supply, ignition switch power supply, and ground circuits. Verify stable voltage (>11V). Verify internal module faults by substitution. After replacement, calibrate the lateral acceleration sensor and initialize the wheel speed sensor signals.
  • 6
    Repair verification and road test: After repairing or replacing the faulty component, clear the fault code and perform a road test (maintain vehicle speed above 40 km/h and drive continuously for more than 5 minutes). Confirm the fault code does not return, the ABS warning light turns off, the left rear wheel speed signal in the data stream matches the other wheel speeds, and the energy recovery function operates normally.
Real-World Cases
BYD DTC AI Analysis

BYD E2 left rear wheel speed sensor intermittent failure after water wading

✓
A 2020 BYD E2 with 32,000 km displayed "Check ABS System" on the instrument cluster. The scan tool retrieved current DTC C1206; freeze frame data showed the fault occurred at 15 km/h. Inspection revealed significant water ingress in the left rear wheel speed sensor connector, with the pins oxidised green. Disassembly revealed the connector seal was missing, likely not refitted during previous repair. Thoroughly cleaned the connector, installed a new waterproof seal, and applied electrical contact protectant. Fault resolved. Check chassis harness waterproofing and avoid driving through water.
BYD DTC AI Analysis

BYD Yuan EV: C1206 False Positive After Tyre Replacement

✓
2019 BYD Yuan EV535. Owner reported ABS warning light on after left rear tyre replacement. Inspection found wheel speed sensor mounting bolt loose; gap between sensor and steering knuckle measured 2.5 mm (standard 0.3–1.0 mm). The technician had not used a torque wrench and left metal debris in the mounting hole. Cleaned the mounting face, adjusted the sensor to set 0.6 mm air gap with the tone ring, and tightened the bolt to 8–10 N·m. Fault cleared. When replacing tyres, protect the sensor from impacts that can displace the internal magnetic core.
BYD DTC AI Analysis

Chassis harness wear caused left rear wheel speed signal open circuit on BYD E3

✓
A commercial BYD E3 rideshare vehicle with 120,000 km: ABS warning light flickered intermittently on rough roads. C1206 stored as a historical code with intermittent open-circuit behavior. On the lift, found severe chafing on the left rear harness near the fuel tank bracket—copper conductors partially broken, only a few strands still connected. Repair: Cut out the damaged section, soldered and heat-shrink sealed. Re-routed and secured the harness, added anti-chafing corrugated tubing to clear sharp body edges. Road-tested one week, no recurrence.
BYD DTC AI Analysis

Internal short circuit in BYD E1 left rear wheel speed sensor caused abnormal ABS pump activation.

✓
2020 BYD E1. ABS pump activated abnormally and frequently while driving, accompanied by a clicking noise; dashboard displayed brake system fault. Read DTC C1206 (current). Measured sensor resistance at only 0.3kΩ (well below the standard 1.2kΩ), determining internal short. Replaced the OEM wheel speed sensor; new sensor measured 1.15kΩ (normal), but the fault code remained. Further inspection revealed the ABS module's corresponding pins had burned from prolonged high current caused by the short. Replaced the left rear wheel speed sensor and ABS control unit assembly. After matching and programming, the fault resolved completely. Note: Sensor shorts can damage the ABS module; perform a thorough inspection.
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.