AU
C05D000

This DTC indicates the signal difference between dual redundant channels A and B of the Master Cylinder Position Sensor inside the IPB (Intelligent Integrated Braking System) exceeds the diagnostic threshold — Seal U

Braking System

This DTC indicates the signal difference between dual redundant channels A and B of the Master Cylinder Position Sensor inside the IPB (Intelligent Integrated Braking System) exceeds the diagnostic threshold.

For functional safety (ISO 26262), the IPB uses a dual-channel redundant design to monitor brake master cylinder piston travel.

When the difference between the two signals continuously exceeds the calibrated value (typically 5%-10%), the system determines the sensor signal is implausible.

This fault triggers the brake system limp-home mode, potentially causing a loss of electronic brake assist, a hard brake pedal, ABS/ESC deactivation, and illuminating multiple system warning lights.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Master cylinder position sensor internal circuit fault or signal drift, causing inconsistent dual-channel outputs.
  • 2Loose mechanical connection between the sensor and master cylinder piston, abnormal clearance, or misaligned installation position.
  • 3Water ingress, oxidation, or loose pins in the IPB wiring harness connector (especially the 32-pin or 48-pin main plug) causing signal crosstalk.
  • 4IPB control unit internal signal acquisition circuit fault (ADC conversion error or power supply interference)
  • 5Brake master cylinder mechanical binding or poor piston return causes a deviation between the actual position and the sensor reading.
What To Do
  • 1
    Use VDS or the BYD dedicated diagnostic tool to read the complete DTC list and freeze frame data. Record the vehicle speed, pedal travel, and A/B channel raw signal values at the time of the fault.
  • 2
    Check if the brake fluid level is normal, and check the IPB and lines for leaks, deformation, or external damage.
  • 3
    Disconnect the IPB wiring harness connector. Check the pins for corrosion, water ingress, or recession. Verify the sensor power supply (5V±0.25V) and ground (<0.1Ω) are normal.
  • 4
    Use an oscilloscope or multimeter to monitor the master cylinder position sensor channel A and B signal voltage curves during pedal travel. Verify both signals change in the same direction and the difference remains within the permitted range.
  • 5
    Check the IPB mechanical mounting bolt torque (usually 8-12 Nm) and verify the connection between the module and the master cylinder is not loose.
  • 6
    If diagnostics confirm a sensor fault and the sensor cannot be replaced separately, replace the complete IPB (Intelligent Integrated Braking System) assembly. The new module must match the vehicle configuration.
  • 7
    Perform the brake system bleeding procedure (use the diagnostic tool to activate motor bleeding combined with conventional bleeding).
  • 8
    Perform master cylinder position sensor zero-point calibration (Master Cylinder Position Calibration) and pedal travel learning.
  • 9
    Clear the fault code, perform static and dynamic tests, and confirm the A/B channel signal difference is within the normal range (<3%).
Real-World Cases
BYD DTC AI Analysis

Song Plus DM-i sensor connector corroded after water ingress

✓
Brake system warning light illuminated after driving through water. Read DTC C05D000. Found water inside the IPB harness connector (located above the left front wheel arch). Pins 3 and 4 (sensor A/B signals) had green corrosion. Cleaned the connector and applied conductive protectant; signal discrepancy temporarily cleared but returned after one week. Replaced the IPB harness connector and waterproofed the connection to completely eliminate the fault.
BYD DTC AI Analysis

Seal master cylinder position sensor internal drift

✓
Customer reported excessive free play at the beginning of brake pedal travel and a "Brake System Fault" warning on the dashboard. Live data showed sensor A voltage rising from 0.5V to 2.8V and sensor B from 0.5V to only 2.1V when depressing the pedal—a 25% difference (normal: <5%). Wiring checked normal, confirming an internal IPB sensor fault. Replaced the IPB assembly and completed calibration. The signal difference between the two channels returned to 1.2%, clearing the fault.
BYD DTC AI Analysis

Wiring harness wear on Qin PLUS caused intermittent fault

✓
ABS warning light illuminated intermittently on rough roads. DTC C05D000 stored as history code. Inspection revealed chafing on IPB-to-body harness transition at firewall mounting clip; Sensor B signal wire insulation worn through, intermittently shorting to shield. Repaired harness and re-secured routing. Fault did not recur over 50 km of continuous road testing.
BYD DTC AI Analysis

Yuan Plus: IPB replaced without calibration, causing fault

✓
The workshop replaced the IPB assembly without performing sensor zero-point calibration. After driving 10 km, the vehicle set fault code C05D000. Live data showed signal values of 0.3 V and 0.8 V for channels A and B respectively at standstill (theoretical value: 0.5 V ± 0.1 V). Executing 'Master Cylinder Position Sensor Zero-point Calibration' and 'Pedal Min/Max Position Learning' with a diagnostic scanner automatically cleared the fault code and restored normal system operation.
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.