AU
C1209

BYD DTC C1209 indicates the ABS system detected an electrical fault in the right rear wheel speed sensor (RR Wheel Speed Sensor) circuit, specifically an open circuit, short to ground, or short to power in the signal line — Atto 8

Braking System

BYD DTC C1209 indicates the ABS system detected an electrical fault in the right rear wheel speed sensor (RR Wheel Speed Sensor) circuit, specifically an open circuit, short to ground, or short to power in the signal line.

In new energy vehicles, the wheel speed signal not only controls ABS/ESC braking but also directly affects the motor regenerative braking strategy, driving range calculation, and automatic emergency braking (AEB) function.

This fault causes a lost or abnormal right rear wheel speed signal, forcing the ABS into fail-safe mode.

This disables anti-lock braking, traction control, and vehicle stability functions, and may trigger regenerative braking derating, creating a safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Open or short circuit in right rear wheel speed sensor internal coil (sensor aging or overheating damage)
  • 2Loose wiring harness connector, backed-out terminals, or seal failure causing water ingress and corrosion (common after chassis water exposure)
  • 3Sensor wiring harness insulation worn through, shorted to body ground, or broken signal wire.
  • 4Incorrect sensor installation gap or damaged tone ring (missing teeth, deformation, contamination) generates an abnormal signal that the system misdiagnoses as a circuit fault.
  • 5ABS control unit internal signal processing circuit fault (less common)
What To Do
  • 1
    Read freeze frame data using the diagnostic tool. Verify vehicle speed and wheel speed status when the fault occurred to rule out intermittent interference.
  • 2
    Raise the vehicle. Visually inspect the right rear wheel speed sensor condition, installation, and wiring harness routing. Check for obvious damage.
  • 3
    Disconnect the sensor connector and measure the sensor resistance (normal value approximately 1.0-1.6 kΩ at 20°C). Infinite resistance indicates an open circuit; 0 or very low resistance indicates a short circuit.
  • 4
    Measure wiring harness continuity between the sensor connector and the ABS control unit to check for an open circuit. Measure the wiring harness insulation resistance to ground and to the power supply to check for a short circuit.
  • 5
    Check the connector pins for oxidation, backing out, or deformation. Clean or replace the connector if necessary.
  • 6
    Remove the sensor and inspect the sensing tip for dirt or damage. Use a feeler gauge to measure the air gap between the sensor and the tone ring (standard value typically 0.3-1.2 mm).
  • 7
    Inspect the tone ring for missing teeth, deformation, or severe contamination (iron filing buildup).
  • 8
    After replacing the faulty sensor or repairing the wiring harness, clear the fault code and perform a road test to verify (accelerate above 40 km/h and observe if all four wheel speeds synchronize).
Real-World Cases
BYD DTC AI Analysis

BYD E2 right rear wheel speed sensor internal short circuit

✓
Dashboard ABS and ESC warning lights remained illuminated. Read DTC C1209. Measured right rear wheel speed sensor resistance at 0.2Ω (normal approximately 1.2kΩ), indicating internal coil short. Replaced right rear wheel speed sensor; new unit measured 1.15kΩ. Cleared fault codes after installation and road tested. All four wheel speed signals normal. Fault resolved.
BYD DTC AI Analysis

BYD Yuan EV wiring harness chafing causes intermittent open circuit

✓
While driving on rough roads, the ABS warning lamp illuminated intermittently. DTC C1209 set with intermittent status. Inspection found the RH rear wheel speed sensor harness chafed through at the underbody shield retaining clip, with copper strands fractured causing poor contact. Stripping back the harness revealed 2 of the 3 wires broken. Resoldered the connections and applied waterproof insulation. The fault did not recur.
BYD DTC AI Analysis

Connector corrosion on BYD E3 after water ingress

✓
After driving through standing water, the ABS warning light illuminated and stored DTC C1209. Inspection found water intrusion and green corrosion inside the right rear wheel speed sensor connector; oxidised pins caused poor signal contact. Cleaned the connector with electrical contact cleaner, applied conductive grease, and replaced the seal. Fault cleared. Recommend checking chassis sealing to prevent further water ingress.
BYD DTC AI Analysis

BYD E1 deformed reluctor ring causing false circuit fault

✓
Impact to the right rear wheel triggered DTC C1209 and illuminated the ABS warning light. Initial inspection found the wheel speed sensor and wiring harness normal, but the diagnostic tool showed an abnormal right rear wheel speed signal. Disassembly revealed the reluctor ring inside the brake drum had deformed from the impact, rubbing against the sensor and causing it to overheat and fail. The reluctor ring's signal teeth were also deformed. Replacing the right rear wheel bearing unit (with reluctor ring) and the wheel speed sensor resolved the fault.
Other Atto 8 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.