AU
C05CB00

This DTC indicates the signal voltage from Master Cylinder Piston Position Sensor A, internal to the Intelligent Integrated Brake System (IPB), falls below the calibrated threshold (typically <0 — Seal U

Braking System

This DTC indicates the signal voltage from Master Cylinder Piston Position Sensor A, internal to the Intelligent Integrated Brake System (IPB), falls below the calibrated threshold (typically <0.5V or below the valid range lower limit).

The sensor uses Hall effect or potentiometer principles to monitor brake master cylinder piston displacement in real time.

It provides the IPB control unit with an accurate pedal travel signal to calculate driver braking demand, coordinate electro-hydraulic brake pressure distribution, trigger regenerative braking, and control active safety functions (ESP/ABS/AEB).

Low voltage indicates a voltage drop in the sensor supply circuit (5V reference voltage), a signal wire short to ground, an internal sensor short circuit, or a faulty IPB control unit sampling circuit.

This fault causes abnormal brake pedal feel, limited or disabled regenerative braking, and degraded ESP/ABS functions.

Extreme conditions trigger brake system fail-safe protection (pure hydraulic braking backup), compromising driving safety.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Damage, wear, or internal short circuit in the master cylinder position sensor A body within the IPB integrated electro-hydraulic brake assembly.
  • 2Water ingress, corrosion, or seal failure at the sensor wiring harness connector (located on the side of the IPB assembly) causing a signal wire short to ground.
  • 3Poor connection, excessive voltage drop, or short to ground in the 5V reference voltage circuit between the IPB control unit and the sensor
  • 4High engine compartment temperatures degraded and damaged the sensor signal wiring harness insulation, causing a short to body ground.
  • 5Hardware fault in the internal signal acquisition circuit or ADC conversion module of the IPB control unit
What To Do
  • 1
    Use VDS or a Launch diagnostic tool to read the IPB live data stream. Verify if the master cylinder position sensor A voltage remains at 0V or <0.5V. Compare with the sensor B value (if equipped) to verify. Check for accompanying DTCs C05CA00 (overvoltage) or C05CC00 (out of range).
  • 2
    Visually inspect the IPB assembly wiring harness connector (usually a grey or black plug located near the brake master cylinder) for looseness, water ingress, pin corrosion, pin recession, or deformation. Clean the connector with electrical contact cleaner, apply conductive grease, and reconnect it. Confirm the locking tab engages fully.
  • 3
    Disconnect the IPB wiring harness connector. Use a multimeter to measure the voltage to ground of the Sensor A 5V reference circuit (usually VREF_A). Verify the voltage is 4.9-5.1V. Measure the resistance to ground of the signal output wire (Signal_A). The resistance must be >10kΩ. If the reading is close to 0Ω, a short to ground exists.
  • 4
    Check the IPB assembly installation status and the tightening torque of the body ground point (usually G102 or G202) (standard: 9-12 N·m). Measure the ground resistance; it should be <1 Ω. Check the IPB power supply fuse (usually 30A or 40A) and voltage drop.
  • 5
    If the wiring tests normal, flash the IPB control unit software (version upgrade) or reset the sensor calibration. If the fault persists, replace the IPB integrated electro-hydraulic brake assembly (the part number depends on the vehicle model; e.g., Song PLUS DMi uses 6A-3505010). After replacement, bleed the brake system, calibrate the sensors, and calibrate the longitudinal acceleration sensor.
Real-World Cases
BYD DTC AI Analysis

IPB harness wear caused signal wire short to ground

✓
A 2021 Song PLUS DM-i with 32,000 km showed "Brake System Fault" on the instrument cluster along with ABS and ESC warning lights. The scan tool read codes C05CB00 (Master Cylinder Position Sensor A Voltage Too Low) and C05D800 (Sensor A/B Consistency Fault). Inspection found the wiring harness retaining clip near the IPB assembly had come loose; the harness had rubbed against the body metal bracket, wearing through the insulation and shorting the signal wire to ground. Repaired the harness by re-wrapping it and fitting corrugated tubing, adjusted the clip position, cleared the codes and calibrated the sensor. Fault resolved.
BYD DTC AI Analysis

Water ingress corroded the IPB connector, causing intermittent undervoltage.

✓
A 2022 Song Pro DMi developed a hard brake pedal and intermittent brake system warnings after driving in heavy rain. Diagnostics showed intermittent DTC C05CB00. Freeze frame indicated the fault triggered at 60 km/h following water wading. Removing the IPB wiring harness connector revealed water inside and green corrosion on pin 1 (Sensor A signal). The connector seal had degraded, allowing water ingress during washing or wading and reducing the signal line resistance to ground. Replaced the IPB wiring harness connector (Repair Kit), cleaned the terminals, and applied waterproof silicone grease. No recurrence.
BYD DTC AI Analysis

IPB internal sensor A damaged; replace assembly.

✓
During PDI of a new Song PLUS DMi, found active DTC C05CB00 that would not clear. Backprobed the IPB connector: 5V reference and sensor B signal (2.3V) were normal, but sensor A signal line read 0.1V and fluctuated. Disconnected the IPB internal plug and measured sensor A resistance directly—signal-to-ground was only 120Ω (normal: 2–5kΩ), confirming the Hall element in sensor A had shorted. The sensor is integrated into the IPB assembly and unavailable separately, so replaced the IPB electro-hydraulic module assembly. Ran the bleed and calibration routines; fault cleared.
BYD DTC AI Analysis

Software calibration error caused false undervoltage fault

✓
A Song PLUS EV developed fault code C05CB00 after a head unit system update. Braking remained normal, but live data showed Sensor A voltage fluctuating between 0.4–0.6V (threshold). Wiring harness and power supply checked normal. Diagnosed as a mismatch between IPB control unit software version and sensor calibration data. Refreshed IPB control unit software to the latest version (3.00.18 or above), then performed Master Cylinder Position Sensor Zero Point Calibration and Brake Pedal Position Learning. Sensor A voltage returned to 2.5V (static) and the fault code cleared automatically.
BYD DTC AI Analysis

High resistance in the reference voltage circuit caused undervoltage in both sensors.

✓
A 2021 Song PLUS DM-i displayed multiple brake system faults on the instrument panel following front-end collision repairs. The scan tool retrieved codes C05CB00 and C05DB00 (Sensor B voltage low). Checked the IPB power supply – normal. Measured the 5V reference voltage for Sensors A and B at only 1.2V. Traced the wiring harness and found the IPB harness pinched during the accident repair. The 5V reference wire (yellow/red) fractured internally, leaving only 2-3 copper strands connected and reducing load capacity. Repaired the harness by soldering the connection and sealing with heat-shrink tubing. Restored the 5V reference voltage and cleared the faults.
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.