AU
C001804

On the BYD E5 and similar new energy vehicles, DTC C001804 indicates a left rear wheel speed sensor (WSS) circuit performance fault, rather than a hydraulic modulator inlet valve fault (early reference materials may contain code definition errors) — Atto 8

Braking System

On the BYD E5 and similar new energy vehicles, DTC C001804 indicates a left rear wheel speed sensor (WSS) circuit performance fault, rather than a hydraulic modulator inlet valve fault (early reference materials may contain code definition errors).

The sensor is typically a variable reluctance or Hall-effect type mounted on the left rear knuckle.

It generates a sine wave or square wave signal by detecting magnetic field changes in the wheel hub bearing magnetic encoder ring, supplying precise wheel speed data to the ABS/ESP control unit.

The ECU sets this DTC when it detects a continuously missing left rear wheel speed signal, out-of-range voltage amplitude, intermittent signal interruption, or an abnormal logical relationship between this wheel speed and the other three (e.g., reading 0 while the vehicle moves).

This fault restricts or completely disables the Anti-lock Braking System (ABS), Electronic Stability Program (ESP), Traction Control System (TCS), Electronic Parking Brake (EPB), and Automatic Emergency Braking (AEB).

On certain 4WD models (such as the Tang DM), it triggers the 4WD protection mechanism, forcing the vehicle into 2WD mode and severely compromising driving safety.

The C001804 fault code on the Atto 8 indicates a left rear wheel speed sensor circuit performance fault. Severity is medium because it can limit or disable ABS, ESP, traction control, EPB, AEB, and related safety functions. Common causes include a failed wheel speed sensor or water ingress, corrosion, or loose terminals at the sensor connector. The first recommended fix is to 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
  • 1Open or short circuit in the wheel speed sensor internal coil (normal magnetic inductive sensor resistance is approximately 1.2-1.8kΩ; values outside this range indicate a faulty sensor).
  • 2Water ingress and oxidation at the sensor wiring harness connector, loose terminals, or poor contact (common after driving through water during the rainy season, washing the vehicle, or due to an aging seal)
  • 3Incorrect sensor installation gap (standard air gap: 0.3-1.0 mm) or loose retaining bolts, resulting in low signal amplitude.
  • 4Iron filings or mud adhering to the surface of the magnetic encoder tone ring (integrated into the wheel hub bearing inner race), or tone ring deformed, demagnetized, or damaged.
  • 5Wheel hub bearing wear causes excessive tone ring radial runout, preventing the sensor from reading a stable magnetic field signal.
What To Do
  • 1
    Connect the VDS2000 or Launch X431 diagnostic tool. Enter the ABS/ESP system to read fault codes. Confirm C001804 is the current fault code. Check the vehicle speed and wheel speed information in the freeze frame data.
  • 2
    Access the live data stream. Slowly push or drive the vehicle and check if the left rear wheel speed signal remains at 0 km/h, fluctuates erratically, or differs from the other three wheel speeds by more than 5 km/h.
  • 3
    Turn off the ignition and disconnect the left rear wheel speed sensor connector. Use a multimeter to measure the resistance between the two terminals on the sensor (normal value for magnetic induction type: 1.2-1.8 kΩ). If the reading shows an open circuit (OL) or short circuit (0 Ω), replace the sensor.
  • 4
    Measure the continuity and insulation to ground of the wiring harness between the sensor connector and the ABS pump ECU. Check for an open circuit, short circuit, or short to ground. Specifically check the wiring harness sleeve for wear points where it passes through the body or suspension.
  • 5
    Visually inspect the sensor plug for an aging or missing sealing ring. Check the terminals for green copper corrosion or burn marks. Clean the terminals with electrical contact cleaner and apply conductive grease. Replace the plug or wiring harness assembly if necessary.
  • 6
    Remove the sensor and inspect the tip for metal debris. Clean foreign matter from the surface of the magnetic encoder tone ring. Use a feeler gauge to check the gap between the sensor and the tone ring. Retighten the sensor to the specified torque (usually 8-12Nm).
  • 7
    If the above checks are normal, raise the vehicle and manually rock the left rear wheel to check the wheel hub bearing radial play. If obvious looseness or abnormal rotational noise is present, replace the wheel hub bearing assembly with a magnetic encoder.
  • 8
    After completing the repair, clear the fault code and drive in a straight line at speeds above 40 km/h for 1-2 minutes to complete sensor auto-calibration. Confirm the instrument cluster warning light turns off and the four wheel speed data streams are consistent.
Real-World Cases
BYD DTC AI Analysis

Wheel speed sensor unit damaged (BYD Song Plus DM-i)

✓
After starting the vehicle, the ESP, ABS and tyre pressure monitoring warning lights illuminate simultaneously on the dashboard. The centre console displays "Please check ESP system". While driving, the vehicle loses electronic stability program intervention, but basic braking remains normal. The warning lights stay on constantly. A VDS2000/Launch X431 scan tool read brake system (ABS/ESP module) DTCs and retrieved C001804 (left rear wheel speed sensor circuit fault). Live data showed the left rear wheel speed reading constantly at 0 km/h or displaying erratic garbled values, while the other three wheel speed signals read normally. The sensor connector showed no looseness. Measuring the sensor resistance (normally approximately 1.2-1.8 kΩ) revealed an open circuit. Solution: Replaced the left rear wheel speed sensor (part number usually starts with 3635100; confirm for specific model year). After installation, cleared the fault codes and drove straight at speeds above 40 km/h for 1-2 minutes to complete the sensor auto-calibration. The warning lights went out and data returned to normal.
Original source ↗
BYD DTC AI Analysis

Wiring harness connector oxidized due to water ingress (BYD Qin Pro DM)

✓
Fault is intermittent. Typically appears 1–2 hours after driving in rain or washing the vehicle, when the dashboard ESP warning light suddenly lights up. Sometimes the light clears after the vehicle sits dry for several hours. False ABS activation also occurs intermittently on bumpy roads. Scanner showed history DTC C001804 and current DTC C001800 (Signal Invalid). Lifted the vehicle and inspected the left rear wheel. The wheel speed sensor connector seal was deteriorated, with obvious green corrosion (oxidation) on the internal pins. Measured continuity from the sensor to the ABS pump and found an intermittent open circuit. Thoroughly cleaned the ABS harness connector and sensor terminals, flushed with electronic cleaner, blown dry, and applied conductive grease. Replaced the waterproof seal and reinforced the wrapping with insulation tape. If internal harness corrosion is severe, replace the left rear wheel speed sensor harness assembly.
Original source ↗
BYD DTC AI Analysis

Incorrect sensor gap and dirty tone ring (BYD Han EV)

✓
At low speeds (10-20 km/h), the driver feels slight kickback through the brake pedal (false ABS activation) accompanied by a clicking noise; alternatively, the vehicle exhibits brief power restriction when pulling away (traction control false activation). The ESP warning light may flash occasionally but does not stay on. Read fault code C001804 (signal plausibility fault). Pulled the left rear wheel speed sensor and found the head covered in iron filings (brake pad wear debris). Inspected the tone ring (integrated into the hub bearing) and found dirt or slight deformation on the surface. Measured the sensor-to-ring gap with a feeler gauge; it exceeded 1.5 mm (standard is 0.3-1.0 mm), likely from previous wheel removal or a loose bearing. Fix: Thoroughly clean the sensor head and tone ring surface. Check the sensor mounting bolts for looseness and retighten to spec (usually 8-12 Nm). If the gap remains excessive, check the hub bearing for play and replace the bearing if necessary to restore the correct clearance.
Original source ↗
BYD DTC AI Analysis

Wheel hub bearing assembly fault (BYD Tang DM)

✓
When driving at high speed (above 80 km/h), a low humming noise comes from the rear and the ESP and four-wheel-drive system warning lights come on. The vehicle may automatically switch from four-wheel drive to two-wheel drive (front-wheel drive), with the instrument cluster displaying "4WD System Function Limited". A scan tool reads DTC C001804 in the ABS system, and the electric 4WD system (MCU) may also log wheel speed signal faults. Lift the vehicle and manually rotate the left rear wheel. Check for noticeable radial play or abnormal noise. Check the left rear wheel speed signal waveform with an oscilloscope. Low amplitude or gaps in the pattern indicate the magnetic encoder ring (tone ring) has radial runout from bearing wear, or the ring is damaged. Solution: Replace the left rear wheel hub bearing assembly (integrated unit with the magnetic encoder ring). After replacement, calibrate the wheel speed sensor. Some models also require a scan tool to initialise and run learn procedures for the EPB (Electric Park Brake) and ESP systems.
Original source ↗
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.