AU
C1202

DTC C1202 indicates that during continuous monitoring, the ABS/ESC control unit (ECU) detects that the left front wheel speed sensor (WSS) input signal remains at 0 km/h or outputs no pulse signal — Seal U

Braking System

DTC C1202 indicates that during continuous monitoring, the ABS/ESC control unit (ECU) detects that the left front wheel speed sensor (WSS) input signal remains at 0 km/h or outputs no pulse signal.

In the Bosch/Continental ABS systems of BYD E1/E2/E3/Yuan series models, this code specifically indicates the left front electromagnetic induction or Hall-effect wheel speed sensor fails to transmit a valid square-wave or sine-wave signal to the ECU.

This causes the ECU to classify the wheel as "stationary" or "signal missing".

This fault triggers the system fail-safe mode, limiting or disabling the Anti-lock Braking System (ABS), Electronic Brakeforce Distribution (EBD), Electronic Stability Control (ESC), Traction Control System (TCS), and Cooperative Regenerative Braking System (CRBS).

This severely impacts vehicle stability and braking safety.

ISO/SAE standards and some Japanese vehicles (such as Toyota) define C1202 as a brake master cylinder reservoir fluid level fault.

However, the fault code system for the specified BYD models clearly maps this code to an abnormal wheel speed sensor signal input.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Open or short circuit in the wheel speed sensor internal coil, or Hall element failure, preventing induced signal generation.
  • 2Water ingress, oxidation, loose connection, or terminal back-out at the sensor wiring harness connector, causing an open signal circuit or excessive contact resistance.
  • 3Abnormal sensor installation gap (gap exceeds 1.2 mm or too small, causing wear), or tone ring (signal disc) deformed, missing teeth, dirty, or jammed by a foreign object.
  • 4Excessive axial/radial play in the left front wheel bearing causes tone ring eccentricity, or suspension component deformation shifts the relative position between the sensor and tone ring.
  • 5ABS control unit internal signal processing circuit fault (abnormal input impedance, damaged sampling circuit, or software signal recognition error)
What To Do
  • 1
    Connect a BYD ED400 or Launch/Autel diagnostic tool. Read the fault codes to confirm whether C1202 is a current or history code. Check if the left front wheel speed value in the data stream remains at 0 km/h, and record the freeze frame data.
  • 2
    Visually inspect the left front wheel speed sensor connector for looseness or water ingress. Check the wiring harness at the fender liner and steering knuckle retaining clips for wear or crush marks. Clean iron filings, mud, and sand from the sensor head and tone ring surface.
  • 3
    Disconnect the sensor connector. Use a multimeter to measure sensor resistance (electromagnetic type: typically 1.0-1.8kΩ; Hall type: supply power before measuring signal voltage) and verify there are no open or short circuits. Measure wiring harness continuity from the sensor to the ABS ECU, and check insulation to ground and power supply.
  • 4
    Raise the vehicle and manually rotate the left front wheel. Measure the sensor output signal waveform using an oscilloscope (should be a valid sine or square wave varying with wheel speed; amplitude increases as speed rises), or measure the induced voltage using a multimeter set to AC voltage (typically >0.1V).
  • 5
    Use a feeler gauge to check the gap between the sensor and the tone ring (standard is usually 0.3-1.2mm; refer to the vehicle repair manual for exact specifications). Check the tone ring for missing teeth or cracks. Shake the tire by hand to check for a loose wheel hub bearing causing tone ring runout.
  • 6
    Perform a sensor swap test: Swap the left front and right front wheel speed sensors (including wiring harness connectors). Clear the fault codes, then road test the vehicle for 1-3 km. Observe if the fault transfers to the right front (C1204 appears) to accurately locate the fault source.
  • 7
    If all above checks are normal, check the ABS control unit power supply, ground, and CAN communication lines. If necessary, reflash the ABS ECU software or replace the ABS ECU.
  • 8
    After repairing or replacing the faulty component, clear the fault code and perform a road test under various conditions (including straight-line acceleration above 40 km/h, emergency braking, and left and right turns). Confirm the ABS/ESC self-check passes, the fault light turns off, and the fault code does not recur.
Real-World Cases
BYD DTC AI Analysis

Left front wheel speed sensor harness chafed at steering knuckle, causing intermittent signal loss

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2020 BYD E2 with 32,000 km. ABS and ESP warning lights illuminated intermittently. Scanner showed current DTC C1202 (left front wheel speed sensor input signal zero). Static test measured sensor resistance at 1.3 kΩ (normal); connector showed no corrosion. Lifted the vehicle to inspect the harness. Found left front wheel speed sensor harness insulation chafed through at the steering knuckle retaining clip from prolonged steering friction, causing copper conductors to short intermittently to ground. Opened the harness: two of three internal wires completely fractured. Repaired the harness by soldering and heat-shrinking. Rerouted the harness to prevent contact with the steering knuckle. Fault eliminated.
BYD DTC AI Analysis

Excessive play in the wheel hub bearing caused the air gap between the tone ring and sensor to exceed specification under dynamic conditions.

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A 2019 Yuan EV developed fault code C1202 with false ABS triggering (pedal kickback) during low-speed braking. Inspection revealed light scratching on the left front wheel speed sensor surface. Static gap measured normal, but rocking the wheel by hand revealed significant axial play in the left front hub bearing (>0.5mm), causing the reluctor ring to wobble eccentrically and the dynamic air gap to exceed the sensor's detection range. Replaced the left front hub bearing assembly (with integrated reluctor ring) and tightened to the specified torque (typically 200–250 N·m). Signal waveform returned to normal. After clearing the fault code, a 10 km road test showed no recurrence.
BYD DTC AI Analysis

Iron filings adhered to the sensor tip, blocking the signal

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A BYD E1 triggered DTC C1202 after maintenance. Inspection found the left front wheel speed sensor head covered in ferromagnetic metal debris (likely from brake pad wear or external ingress), blocking the magnetic circuit between the sensor and tone ring and preventing effective detection. Cleaned the sensor head and tone ring, blew out the mounting hole with compressed air, and reinstalled the sensor. Data stream showed the wheel speed signal returned to normal. Inspect brake pad wear to confirm no abnormal metal shedding.
BYD DTC AI Analysis

Reference Case: Brake Master Cylinder Fluid Level Switch Circuit Fault (C1202 Technical Reference for Toyota Models)

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Note: No public repair records for BYD C1202 exist. This case uses Toyota/Lexus data to illustrate another common ISO definition of C1202. The ABS/ESP warning lights stayed on constantly. The scanner showed C1202 (Master Reservoir Level Malfunction). Brake fluid level was normal, but the fluid level warning switch signal circuit—integrated in the brake master cylinder with booster assembly—shorted to ground. Inspection found oxidation at the A43 connector pins causing poor contact. Replaced the brake master cylinder with booster assembly, performed linear solenoid offset learning, and calibrated the yaw rate and acceleration sensor zero points. Fault cleared. If a BYD vehicle shows C1202 and wheel speed checks are normal, check the brake fluid level and sensor circuit using this logic.
Original source ↗
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.