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 — Atto 8

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 Atto 8 indicates a low voltage signal from Master Cylinder Piston Position Sensor A inside the IPB brake system. This is a medium severity issue that can affect brake feel, regenerative braking, and ABS/ESP performance. Common causes include an internal sensor short or water/corrosion at the IPB connector causing a short to ground. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Damage, wear, or internal short circuit of the master cylinder position sensor A body inside 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 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 bay 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. Confirm 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, water ingress, pin corrosion, backed-out pins, or deformation. Clean the connector with electrical contact cleaner, apply conductive grease, and reconnect. Verify the locking tab fully engages.
  • 3
    Disconnect the IPB wiring harness connector. Use a multimeter to measure the voltage between the Sensor A 5V reference circuit (usually VREF_A) and ground. Verify the voltage is 4.9-5.1V. Measure the resistance between the signal output wire (Signal_A) and ground. The resistance should be >10kΩ. A reading close to 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; the value must be < 1 Ω. Check the IPB power supply fuse (usually 30 A or 40 A) and voltage drop.
  • 5
    If circuit measurements are 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 (part number depends on vehicle model, e.g., 6A-3505010 for Song PLUS DMi). After replacement, bleed the brake system and perform 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 Atto 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.