AU
C05CE00

This DTC indicates the master cylinder piston position sensor B circuit voltage in the IPB (Intelligent Power Brake) smart integrated braking system falls below the calibrated threshold (typically <0 — Seal U

Braking System

This DTC indicates the master cylinder piston position sensor B circuit voltage in the IPB (Intelligent Power Brake) smart integrated braking system falls below the calibrated threshold (typically <0.5V).

The master cylinder position sensor uses a dual-sensor redundant design (sensors A and B) to monitor brake master cylinder piston displacement in real time.

It provides brake pedal travel signals to the IPB ECU to calculate driver braking intent and control electro-hydraulic brake assist.

Low sensor B voltage prevents the IPB system from accurately identifying the master cylinder position.

This triggers the system safety protection mechanism and limits or disables ESP/ABS functions, potentially causing abnormal brake pedal travel, reduced brake assist, or complete loss of brake assist, seriously compromising driving safety.

A short to ground in the sensor power or signal circuit, poor wiring contact, or a damaged sensor typically causes this fault.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Poor contact at the sensor wiring harness connector, oxidized pins, backed-out pins, or corrosion from water ingress, causing excessive signal transmission resistance.
  • 2Master cylinder position sensor B power supply circuit (5V reference voltage) short to ground or open circuit, causing insufficient operating voltage.
  • 3A short to ground in the sensor signal output circuit causes the ECU to detect an abnormally low voltage.
  • 4Brake master cylinder position sensor B internal short circuit damage (sensor integrates into the master cylinder assembly; do not replace separately).
  • 5IPB control unit (ECU) internal signal processing circuit fault or abnormal software calibration
What To Do
  • 1
    Use a dedicated diagnostic tool (such as BYD VDS) to read the DTC freeze frame data and confirm the vehicle status and sensor voltage values when the fault occurred.
  • 2
    Disconnect the IPB wiring harness connector and visually inspect the pins for oxidation, recession, corrosion, or deformation. Clean or repair the connector.
  • 3
    Disconnect the master cylinder position sensor B connector and measure the power supply pin voltage (should be 5V±0.25V). If abnormal, check the wiring harness for a short to ground or open circuit.
  • 4
    Measure the sensor signal wire voltage (normally 0.5-4.5V at idle, varying with pedal travel). If the voltage remains below 0.5V, check the signal wire for a short to ground.
  • 5
    Measure the sensor resistance (compare with the standard value in the workshop manual, usually several kilo-ohms). If the resistance is abnormal, the sensor is faulty.
  • 6
    Check wiring harness continuity (between the sensor and the IPB ECU) and insulation to ground. Repair any damaged or shorted wiring.
  • 7
    If the wiring harness is normal but sensor parameters are abnormal, replace the brake master cylinder assembly with sensor (the sensor is not available separately).
  • 8
    After replacement, perform the IPB brake system bleeding procedure to remove air from the lines.
  • 9
    Use the diagnostic tool to perform the 'Master Cylinder Position Sensor Learning' or 'IPB Calibration' procedure to complete the sensor zero-point calibration.
  • 10
    Clear the fault code, perform a road test to verify braking performance, and confirm the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Oxidized pins caused excessive contact resistance in the Seal 06 DM-i

✓
ESP warning light on. Brake pedal soft, travel increased. Scanned with VDS: IPB DTC C05CE00. Disconnected master cylinder position sensor B connector. Pins showed severe oxidation and green corrosion; contact resistance measured >5Ω (normal <1Ω). Cleaned pins and applied conductive grease. Sensor supply voltage restored to 5V; signal voltage varies 0.5-4.5V with pedal travel. Cleared codes and road tested; fault eliminated. Recommend checking for water immersion history and installing waterproofing.
Original source ↗
BYD DTC AI Analysis

Atto 3 wiring harness crushed and damaged, causing short to ground

✓
Owner reported the brake system warning light illuminates intermittently after starting. Scanner showed alternating codes C05CE00 and C05CD00 (Sensor B voltage too high). Inspection revealed the master cylinder position sensor B harness sheath damaged at the engine compartment firewall; a metal bracket had crushed the internal wires, wearing through the insulation and creating an intermittent short to ground. Repaired the damaged harness and rerouted the wiring to avoid contact. Signal line insulation resistance to ground measured >10MΩ. Replaced the brake master cylinder assembly due to potential sensor damage from the short, then performed the sensor calibration procedure. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Song PLUS DM-i discharged 12V battery caused false DTCs

✓
Vehicle displayed "Please check brake system" after parking in cold conditions. Scan revealed historical DTC C05CE00; current sensor voltage read normal. Battery voltage was only 11.2V (normal >12V). The IPB system can trigger false diagnostics under low voltage. Charged the battery and checked state of health—SOH only 65%. Replaced the battery and cleared all historical codes with a scan tool. Flashed the IPB control unit to the latest software (optimizes low-voltage threshold judgment). Performed complete brake system calibration and bleeding. Monitored for one week with no recurrence. Confirmed: intermittent false alarm caused by low voltage.
Original source ↗
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.