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 3

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 3
  • 1Open or short circuit in right rear wheel speed sensor internal coil (sensor aging or overheating damage)
  • 2Loose wiring harness connector, terminal back-out, or seal failure causing water ingress and corrosion (common after chassis wading)
  • 3Sensor wiring harness chafed, shorted to body ground, or signal wire broken.
  • 4Incorrect sensor installation gap or tone ring damage (missing teeth, deformation, contamination) causes an abnormal signal, resulting in a false circuit fault diagnosis.
  • 5ABS control unit internal signal processing circuit fault (less common)
What To Do
  • 1
    Use the diagnostic tool to read freeze frame data. Confirm 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 appearance, installation condition, and wiring harness routing. Check for obvious damage.
  • 3
    Disconnect the sensor connector and measure the sensor resistance (normal value approx. 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 wiring harness insulation resistance to ground and to the power supply to check for a short circuit.
  • 5
    Check the connector pins for oxidation, backed-out pins, or deformation. Clean or replace the connector if necessary.
  • 6
    Remove the sensor and inspect the sensing tip for dirt or damage. Measure the air gap between the sensor and the tone ring using a feeler gauge (standard value typically 0.3-1.2 mm).
  • 7
    Check the tone ring for missing teeth, deformation, or severe contamination (metal filings build-up).
  • 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 the speed data for all four wheels synchronizes).
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 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.