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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) — Seal U

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.

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Cases Logged
Real-World Cases
BYD DTC AI Analysis

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

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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)

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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)

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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)

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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.
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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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.