AU
C050400

DTC C050400 indicates the IPB (Intelligent Integrated Braking System) control unit detects an "indirect uncertainty" (plausibility fault) in the left front wheel speed sensor signal — Atto 8

Braking System

DTC C050400 indicates the IPB (Intelligent Integrated Braking System) control unit detects an "indirect uncertainty" (plausibility fault) in the left front wheel speed sensor signal.

This is not a simple sensor open or short circuit.

Instead, the control unit algorithm identifies a logical inconsistency between the left front wheel speed signal and the vehicle's actual motion state, the speeds of the other three wheels, and the longitudinal/lateral acceleration sensor data.

Symptoms include signal spikes, an abnormal fixed value (such as a constant 1845 km/h), a signal rate of change exceeding physical limits, or intermittent signal loss within specific vehicle speed ranges (usually >60 km/h).

This fault triggers degraded protection modes in systems including ABS, ESP, and automatic emergency braking, severely affecting vehicle dynamic stability control.

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Cases Logged
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Causes
Likely Causes — Atto 8
  • 1Magnetic encoder target wheel contamination: Iron filings (brake pad wear dust) or oil accumulated on the half shaft magnetic encoder surface distorts the magnetic field signal, causing abnormal square waves or signal loss.
  • 2Wheel speed sensor aging: Hall element performance degrades, causing insufficient output signal amplitude. The control unit cannot correctly identify the signal at high rotational speeds.
  • 3Intermittent wiring harness fault: Left front wheel speed sensor harness shorts or opens intermittently during steering or suspension travel, or oxidized connector pins or backed-out terminals increase contact resistance.
  • 4Abnormal installation gap: The air gap between the sensor and tone wheel falls outside the standard range (0.2-1.2mm) due to collision, bushing wear, or mounting bracket deformation, resulting in insufficient signal strength.
  • 5Electromagnetic interference: Interference from the high-voltage system (drive motor, DC-DC) couples into the wheel speed signal line, or damaged sensor shielding causes signal distortion.
What To Do
  • 1
    Initial diagnosis: Use the VDS diagnostic tool to read the complete fault code freeze frame. Record the vehicle speed and wheel speed data from when the fault occurred. Enter "Live Data Stream" mode and observe the four wheel speed signals. Check whether the left front wheel shows 0 km/h, a fixed abnormal value (such as 1845km/h), or a significant difference from the other wheels.
  • 2
    Basic check: Turn off the ignition switch. Inspect the left front wheel speed sensor connector (usually located near the shock absorber) for looseness, water ingress, or pin oxidation. Measure the sensor resistance (Hall type: approx. 1.0-1.5kΩ; magnetic induction type: approx. 0.8-1.4kΩ). Check the wiring harness for continuity and insulation to ground.
  • 3
    Target wheel inspection (critical step): Raise the vehicle, remove the left front tire, and clean the sensor head. Remove the left front half shaft. Thoroughly inspect the magnetic encoder target wheel surface for metal debris, scratches, or magnetic pole damage. Clean the target wheel with a clean, soft cloth and alcohol. Never use metal tools to scrape the target wheel.
  • 4
    Air gap and waveform check: Use a feeler gauge to check the air gap between the sensor and the tone wheel (standard: 0.2-1.2mm). If available, use an oscilloscope to measure the sensor output signal waveform. A normal waveform displays a 0V/12V square wave (Hall type) or sine wave (magnetic inductive type). Check for missing signals, noise, or abnormal amplitude.
  • 5
    Dynamic verification: Clear the fault code and road test the vehicle, focusing on speeds above 60 km/h and steering maneuvers. If the fault is intermittent, simulate suspension movement (raise the vehicle, engage a gear to rotate the wheels, and simultaneously shake the wiring harness) to check for intermittent poor contact. If the fault persists, replace the wheel speed sensor with an OEM part and recalibrate it (some models require sensor calibration).
Real-World Cases
BYD DTC AI Analysis

Song PLUS DM-i ABS light on during high-speed driving; wheel speed signal stuck at 1845km/h

✓
Symptoms: Engine and ABS warning lights illuminated intermittently while driving, readily reproduced above 60 km/h. Once triggered, the left front wheel speed signal froze at 1845km/h. Diagnosis: Scan tool retrieved P121B00 (Left Front Wheel Speed Sensor Signal Abnormal) from CEMS and C050400 (Left Front Wheel Speed Sensor Signal Fault) from IPB. Disconnected the sensor connector and inspected the ABS plug pins; no abnormalities. Replaced the sensor but the fault persisted. Removed the driveshaft and found the magnetic encoder ring coated in iron filings. Solution: Cleaned the iron filings from the encoder ring, reinstalled the driveshaft, and test drove. Fault eliminated. Key point: You cannot see the internal debris by removing only the wheel speed sensor; you must remove the driveshaft to clean it thoroughly.
Original source ↗
BYD DTC AI Analysis

BYD All-Model Wheel Speed Sensor General Diagnostic Procedure (Official Technical Bulletin)

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Symptoms: Dashboard ABS/ESP warning lights on, ESP function limited, possible abnormal ABS activation while driving. Diagnostic procedure: 1. Used VDS to read ESP/ABS DTCs, confirmed C050400. 2. Checked left front wheel speed sensor harness connections and resistance (normal: 1.0–1.5 kΩ). 3. Checked installation gap (0.2–1.2 mm). 4. Inspected reluctor ring for metal debris, oil, or damage. 5. Used oscilloscope to check wheel speed sensor signal for interruptions or abnormalities. Resolution: Clean or replace left front wheel speed sensor; clean or replace half-shaft magnetic encoder ring; repair wiring harness short/open; replace half-shaft assembly if necessary.
BYD DTC AI Analysis

Song/Qin Series Multi-System Warning (ESP/ABS Fault)

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Symptoms: While driving, the instrument panel suddenly displayed "Check ESP System" and "Check ABS System" warnings, accompanied by multiple system alerts. Diagnosis: Scanned the vehicle using VDS1000 and retrieved C050400 (Left Front Wheel Speed Sensor Signal Fault) from the ESP module. Found slight oxidation on the wheel speed sensor connector. Resistance measured within normal range but voltage signal was unstable. Further inspection revealed abnormal wear marks between the sensor and the tone ring. Resolution: Replaced the left front wheel speed sensor, cleaned the connector and applied conductive grease, and removed oxides from the tone ring surface. Cleared the fault codes and road tested 20 km. Fault did not reappear.
Original source ↗
BYD DTC AI Analysis

Intermittent brake system fault in new energy vehicles (intermittent short circuit in wiring harness)

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Symptoms: BYD Tang/Han models intermittently display "Check Brake System" on the instrument cluster while driving, with ESP function restricted and abnormal vehicle speed signals. Diagnosis: Scanned the ABS/ESP system and retrieved DTC C050400. Data comparison revealed intermittent fluctuations in the left front wheel speed signal. Raised the vehicle and found the left front wheel speed sensor harness rubbing against the fender liner at full steering lock; worn insulation caused an intermittent short to ground. Inspection also found metal powder from brake pad wear covering the target wheel surface. Repair: Repaired the damaged harness and rerouted it to avoid interference. Thoroughly cleaned the left front half-shaft magnetic encoder target wheel (removed the half-shaft for cleaning). Replaced the damaged harness protective sleeve.
Other Atto 8 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.