AU
C1201

DTC C1201 indicates the ABS control unit detects that the left front wheel speed sensor signal's rate of change or amplitude exceeds the calibrated threshold — Seal U

Braking System

DTC C1201 indicates the ABS control unit detects that the left front wheel speed sensor signal's rate of change or amplitude exceeds the calibrated threshold.

Specifically, the sensor's square-wave output signal exhibits sudden frequency changes, intermittent interruptions, or abnormal voltage fluctuations, preventing the ABS module from accurately calculating the real-time left front wheel speed.

This fault can trigger false ABS activation (pump operates without braking input), restrict Electronic Stability Program (ESP) functionality, disable Automatic Emergency Braking (AEB), and in extreme cases, limit power output on some BYD new energy vehicles equipped with torque vectoring control.

Classified as a "plausibility fault," it differs from open or short circuit faults by focusing on dynamic signal quality monitoring.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Inter-turn short circuit in the internal coil of the front left wheel speed sensor, or magnetic core aging, causing signal amplitude attenuation.
  • 2The air gap between the sensor and the reluctor ring exceeds the standard range (normally 0.3-0.8mm), usually due to excessive axial play in the wheel hub bearing or a deformed sensor bracket.
  • 3Ferromagnetic debris (such as brake dust or metallic road debris) adhering to the tone ring surface, or tone ring deformation or missing teeth.
  • 4Sensor wiring harness develops an intermittent open or short circuit during steering or suspension travel. A detached harness retaining clip commonly allows the harness to rub against the tire or suspension components.
  • 5Abnormal sampling in the ABS control unit internal signal processing circuit (less common; rule out external factors before confirming)
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS or X-431) to read the complete DTCs and freeze frame data. Confirm the vehicle speed and ABS operating status at the time of the fault. Check for an accompanying C1200 (circuit fault) or other wheel speed sensor fault codes.
  • 2
    Raise the vehicle. Inspect the left front wheel speed sensor for physical damage, the connector for oxidation or water ingress, and the wiring harness for abrasion. Clean metal shavings from the sensor head.
  • 3
    Disconnect the sensor connector and measure the sensor resistance (standard value: 1.0-1.6 kΩ at 20°C). Check for infinite resistance (open circuit) or near 0 Ω (short circuit). Measure the insulation resistance to ground on the wiring harness side; the value must be greater than 10 MΩ.
  • 4
    Use a feeler gauge to check the radial clearance between the sensor head and the tone ring (0.3-0.8 mm). Rotate the wheel to check the tone ring radial runout (< 0.1 mm) and tooth profile integrity.
  • 5
    Connect an oscilloscope to monitor the sensor signal waveform. A normal signal is a square wave with an amplitude of 7-14V and a frequency that changes linearly with wheel speed. Compare the signal with the right front wheel waveform. If the waveform is missing, has low amplitude, or shows noise interference, replace the sensor.
  • 6
    If the sensor is normal, check the wheel hub bearing axial play (measure with a dial indicator; standard value ≤0.05mm). If the play is excessive, replace the wheel hub bearing assembly. Finally, perform a road test to confirm the fault code does not return and the ABS functions normally.
Real-World Cases
BYD DTC AI Analysis

BYD E2 ABS light on after driving through water, C1201 intermittent fault

✓
2020 BYD E2, 28,000 km. Owner reported the ABS warning light came on intermittently after rain. Connected a VDS2000 scan tool and retrieved DTC C1201 (current fault). Freeze frame showed the fault triggered at approximately 15 km/h. Found the left front wheel speed sensor head covered in mud, water, and iron debris. Cleaned the sensor and the fault disappeared during the road test. Also found the sensor wiring harness retaining clip had fallen off, causing the harness to chafe the inner tire wall during steering and wear through the insulation. Thoroughly cleaned the sensor and tone ring, repaired the wiring insulation, re-secured the harness clip, applied waterproof silicone grease. Fault resolved.
BYD DTC AI Analysis

Worn left front wheel bearing caused DTC C1201 on Yuan EV

✓
2019 BYD Yuan EV with 65,000 km. ABS and ESP warning lights constantly illuminated on the instrument panel. DTC C1201 (current) retrieved. Sensor resistance measured normal (1.2 kΩ), but feeler gauge measurement showed the gap between the sensor and tone ring varied cyclically between 0.1 mm and 1.5 mm. Inspection revealed obvious axial play in the left front wheel hub bearing (approximately 0.3 mm), causing excessive radial runout of the tone ring during rotation and fluctuating sensor signal amplitude beyond threshold. Replaced the left front wheel hub bearing assembly (including tone ring), restoring the gap to 0.5 mm. Test drove 20 km; fault did not recur.
BYD DTC AI Analysis

BYD E3 wheel speed sensor signal drift from internal aging

✓
A 2021 BYD E3 at 42,000 km developed intermittent false activation of the ABS pump at low speeds (10–30 km/h) with stored DTC C1201. Static resistance measurements were normal, but dynamic oscilloscope monitoring revealed intermittent amplitude drops in the left front wheel speed signal at low speeds (falling from 12 V to 3 V) while other wheel signals remained stable. Replaced the left front wheel speed sensor; signal amplitude stabilized at 11–14 V and road testing confirmed the ABS false activation stopped. Root cause: insulation degradation of the sensor’s internal coil from prolonged high-temperature exposure (proximity to the brake disc), causing insufficient induced electromotive force at low speeds.
BYD DTC AI Analysis

Aftermarket wheels caused the tone ring to interfere with the sensor

✓
2020 BYD e2. Owner fitted aftermarket wheels and drove ~500 km before the ABS warning light illuminated with code C1201. Inspection revealed the wheels’ center bore matched the hub, but the rear face design differed from factory specifications, shifting the brake disc mounting position and moving the tone ring inward ~1.2 mm. The tone ring rubbed the wheel speed sensor, leaving wear marks on the sensor tip and scratches on the tone ring surface. Solution: Install factory-specification wheels or add wheel spacers to restore original mounting dimensions, and replace the damaged wheel speed sensor.
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.