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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.

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Cases Logged
Real-World Cases
BYD DTC AI Analysis

BYD E2 right rear wheel speed sensor internal short circuit

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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

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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

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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

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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
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
Data confidence: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.