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 — Atto 8

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.

The C1201 fault code on the Atto 8 indicates a medium-severity problem with the left front wheel speed sensor signal. The ABS control unit is detecting sudden signal changes, dropouts, or abnormal voltage, which can affect ABS, ESP, and AEB operation. Common causes include a failing sensor coil or magnetic core, or an incorrect sensor-to-reluctor air gap from bearing play or bracket damage. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Inter-turn short circuit in the left front wheel speed sensor internal coil, or magnetic core aging, causing signal amplitude attenuation.
  • 2The air gap between the sensor and the reluctor ring exceeds the standard range (normal: 0.3-0.8 mm), usually due to excessive wheel hub bearing axial play or a deformed sensor bracket.
  • 3Ferromagnetic debris (such as brake wear dust or road metal fragments) adhering to the tone ring surface, or missing teeth or deformation on the tone ring.
  • 4The sensor wiring harness develops an intermittent open or short circuit during steering or suspension travel. This commonly occurs when a detached harness retaining clip allows the harness to rub against the tire or suspension components.
  • 5Abnormal sampling in the ABS control unit internal signal processing circuit (less common; confirm after ruling out external factors)
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (VDS or X-431) to read complete DTCs and freeze frame data. Confirm vehicle speed and ABS operating status at the time of the fault, and check for accompanying C1200 (circuit fault) or other wheel speed sensor fault codes.
  • 2
    Raise the vehicle. Check the left front wheel speed sensor for physical damage, the connector for oxidation or water ingress, and the wiring harness for signs of 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 an infinite reading (open circuit) or a reading near 0 (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 tip and the tone ring (0.3-0.8 mm). Rotate the wheel to check the tone ring radial runout (less than 0.1 mm) and tooth profile integrity.
  • 5
    Connect an oscilloscope to monitor the sensor signal waveform. A normal signal is a 7-14V square wave with a frequency that changes linearly with wheel speed. Compare this with the right front wheel waveform. Replace the sensor if the waveform is missing, has low amplitude, or shows noise interference.
  • 6
    If the sensor is normal, measure the wheel hub bearing axial play using a dial gauge (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

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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

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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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.