AU
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 — Atto 8

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
5
Causes
Likely Causes — Atto 8
  • 1Wheel speed sensor internal coil open or short circuit: Prolonged exposure to high temperatures and vibration can cause the internal copper winding to break from thermal fatigue or damage the insulation layer, creating 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: The chassis wiring harness may rub against the body bracket over bumps, damaging the outer insulation and causing an open circuit or intermittent poor contact. Aging connector seals allow water ingress, causing pin oxidation and corrosion (especially common after water wading or in high-humidity environments) and interrupting signal transmission.
  • 3Abnormal 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). Metal shavings or mud on the tone wheel teeth, tone wheel deformation, or missing teeth cause insufficient signal amplitude, resulting in a false circuit fault.
  • 4ABS control module internal circuit fault: A damaged sampling resistor or filter capacitor in the internal signal processing circuit, or a cold solder joint at the connector, prevents the module from correctly identifying sensor signals and triggers a false open/short circuit fault.
  • 5High-voltage electromagnetic interference (specific to New Energy Vehicles): Damaged shielding or poor grounding in the high-voltage system (drive motor, DC-DC) of BYD pure electric models can generate electromagnetic interference. This interference 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 stream and confirm if C1206 is a current fault (Active). 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℃). Infinite resistance indicates an open circuit. Resistance near 0Ω indicates a short circuit. Replace the sensor if the resistance deviates significantly from the standard value. Measure the insulation resistance between the sensor and the vehicle body ground. The resistance must exceed 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 ABS module pins (usually pins 18# and 33#; refer to the vehicle wiring diagram). Resistance must be less than 1Ω. Measure the insulation resistance of this circuit to body ground and the 12V power supply to confirm 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. With the vehicle raised, use an oscilloscope or a multimeter set to AC voltage. Manually rotate the left rear wheel and observe the signal output. A magnetic induction sensor should output a sine-wave AC voltage that increases with wheel 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 any 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, ignition switch power supply, and ground circuits. Ensure 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 a vehicle speed above 40 km/h for more than 5 minutes). Verify 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 Atto 8 Fault Codes
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.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.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.