AU
C1203

DTC C1203 indicates the ABS detects an electrical fault in the right front wheel speed sensor (WSS) circuit, specifically a signal line short to power/ground or open circuit — Seal U

Braking System

DTC C1203 indicates the ABS detects an electrical fault in the right front wheel speed sensor (WSS) circuit, specifically a signal line short to power/ground or open circuit.

The sensor typically uses Hall effect or electromagnetic induction principles, providing pulse signals to the ABS control unit (ECU) to calculate wheel speed.

The ECU triggers this fault when it detects a continuously abnormal signal voltage (e.g., 0V or 12V, instead of the normal 0.5-4.5V fluctuation range) or fails to receive pulse signals from the passing tone ring.

This causes active safety systems such as ABS, ESC, and EBD to fail or enter a degraded mode.

In severe cases, it may affect the coordinated operation of the energy recovery system because the vehicle control system cannot accurately determine the real-time speed and slip ratio of the right front wheel.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Internal open or short circuit in the sensor body: Damaged Hall element, burned-out coil, or internal circuit board failure due to water ingress or high temperature, resulting in a constant output signal or complete loss of signal.
  • 2Wiring harness connector fault: Water ingress or mud contamination at the right front fender liner sensor plug causes pin oxidation, backed-out pins, or seal failure, resulting in intermittent poor contact or a short circuit.
  • 3Wiring insulation damage: Long-term vibration and chafing damage the sensor wiring harness insulation at the steering knuckle, suspension control arm, or body pass-through, causing a short to body ground or an adjacent power wire.
  • 4Tone ring (signal disc) fault: The tone ring inside the right front driveshaft or wheel hub bearing is deformed, cracked, heavily contaminated (metal filings attached), or incorrectly installed. This prevents the sensor from detecting valid magnetic field changes, causing the ECU to falsely detect a circuit fault.
  • 5Excessive gap: Wheel hub bearing wear causes the air gap between the sensor and tone ring to exceed the standard value (usually 0.3-1.2mm). The system identifies the resulting low signal amplitude as an open circuit.
What To Do
  • 1
    Initial inspection: Raise the vehicle. Visually inspect the right front wheel speed sensor connector (usually located near the shock absorber or steering knuckle) for looseness, water ingress, or corrosion. Inspect the wiring harness retaining clips at the fender liner and suspension moving parts for detachment and signs of wear.
  • 2
    Sensor body inspection: Disconnect the connector and use a multimeter to measure sensor resistance (electromagnetic type is usually 1-2kΩ; Hall type requires power to test). Compare the reading with the left front wheel sensor data. Check the sensor tip for metal shavings or physical damage.
  • 3
    Circuit continuity and insulation test: Measure circuit continuity from the ABS pump ECU connector to the right front sensor connector (CAN-H and CAN-L, or signal/power/ground wires). Measure insulation resistance to body ground (should be >10MΩ) to rule out an internal short circuit in the wiring harness.
  • 4
    Tone ring and gap inspection: Rotate the right front wheel and inspect the tone ring on the inner side of the driveshaft or wheel hub for missing teeth, cracks, or foreign objects. Measure the gap between the sensor and the tone ring using a feeler gauge. Replace the wheel hub bearing assembly if necessary.
  • 5
    Dynamic verification and clearing: After repairing or replacing components, clear the fault code. Drive straight and perform left and right steering tests at 20-40 km/h. Read the four-wheel speed data stream using the diagnostic tool. Confirm the right front wheel speed matches the other wheel speeds and shows no jumps.
Real-World Cases
BYD DTC AI Analysis

Oxidized right front wheel speed sensor connector on BYD E2 caused intermittent short circuit

✓
2019 BYD E2, 30,000 km. Intermittent ABS and ESC warning lights on the instrument cluster, increasing in wet weather. Diagnostic tool read C1203, and live data showed the right front wheel speed intermittently dropping to zero. Inspection found severe oxidation (green corrosion) on the internal pins of the right front wheel speed sensor connector—likely water ingress from wading through flooded roads due to poor sealing. Cleaning the connector, applying conductive grease, and replacing the waterproof seal cleared the fault.
BYD DTC AI Analysis

BYD Yuan EV Wheel Hub Bearing Wear Causing C1203 and Abnormal Noise

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The 2019 Yuan EV developed a metallic grinding noise from the right front wheel when cornering, with the ABS warning light constantly on and DTC C1203. Inspection revealed excessive play in the right front hub bearing (radial runout exceeded 0.5 mm), causing the clearance between the wheel speed sensor and tone ring to vary. Replaced the hub bearing assembly (including tone ring). Clearance returned to specification, signal stabilized, fault code cleared, and noise disappeared.
BYD DTC AI Analysis

BYD E3 wiring harness chafing caused right front wheel speed signal to short to ground

✓
A 2020 E3 developed a C1203 DTC after service at the dealership. Investigation traced the fault to the right front wheel speed sensor harness being pinched against the fender liner edge during vehicle lifting, damaging the insulation and grounding the copper core to the body. Re-wrapped and rerouted the harness to avoid contact with sharp metal edges; fault resolved.
BYD DTC AI Analysis

BYD Song Pro right front wheel speed sensor internal Hall element failure

✓
The ABS warning light suddenly illuminated while driving. DTC C1203 was present as a current fault. Measured sensor supply (12V) and ground — normal, but the signal line showed no pulse output. Replaced the right front wheel speed sensor; the signal waveform (square wave) returned to normal. Dissected the old sensor and found the internal Hall chip burned out due to overheating, likely from severe vibration and high temperatures during previous off-road driving.
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.