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 — Qin Plus

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.

The C05CB00 fault code on the Qin Plus indicates low voltage from Master Cylinder Piston Position Sensor A inside the IPB brake system. Severity is medium because braking assistance, regenerative braking, ABS, or ESP functions may be reduced. Common causes include an internal sensor short or water/corrosion in the IPB sensor connector causing a short to ground. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Damage, wear, or internal short circuit in the master cylinder position sensor A body within the IPB integrated electro-hydraulic brake assembly.
  • 2Sensor wiring harness connector (located on the side of the IPB assembly): Water ingress, corrosion, or seal failure causing the signal wire to 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
  • 4Sensor signal wiring harness insulation aged and damaged by high engine compartment temperatures, causing a short to body ground.
  • 5Hardware fault in the IPB control unit internal signal acquisition circuit or ADC conversion module
What To Do
  • 1
    Use a VDS or 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 the value with sensor B (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 located near the brake master cylinder, grey or black plug) for looseness, signs of water ingress, terminal corrosion, recessed pins, or deformation. Clean with electrical contact cleaner, apply conductive grease, and reconnect. Confirm the locking tab fully engages.
  • 3
    Disconnect the IPB wiring harness connector. Use a multimeter to measure the voltage to ground on Sensor A's 5V reference voltage circuit (usually VREF_A) and verify it is in the 4.9-5.1V range. Measure the resistance to ground on the signal output wire (Signal_A). The resistance should be >10kΩ. A reading near 0Ω indicates a short to ground.
  • 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 must be <1 Ω. Check the IPB power supply fuse (usually 30A or 40A) and voltage drop.
  • 5
    If circuit measurements are normal, perform an IPB control unit software flash (version upgrade) or sensor calibration reset. If the fault persists, replace the IPB integrated electro-hydraulic brake assembly (part number varies by vehicle model, e.g., Song PLUS DMi: 6A-3505010). After replacement, perform brake system bleeding, sensor calibration, and longitudinal acceleration sensor calibration.
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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus overview →
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.