AU
C05CD00

DTC C05CD00 indicates the signal voltage of master cylinder piston position sensor B inside the IPB (Intelligent Integrated Braking System) continuously exceeds the calibrated threshold (typically >4 — Atto 8

Braking System

DTC C05CD00 indicates the signal voltage of master cylinder piston position sensor B inside the IPB (Intelligent Integrated Braking System) continuously exceeds the calibrated threshold (typically >4.8V).

This Hall-effect position sensor monitors the brake master cylinder piston travel position in real time.

It provides the IPB with an accurate brake pedal input signal to enable brake-by-wire assist, coordinated energy recovery, and ESC intervention.

This high voltage indicates a short to power in the sensor signal circuit, an internal sensor breakdown short, or an IPB control module sampling circuit fault.

This fault forces the IPB into degraded mode, which causes abnormal brake assist (stiff pedal or altered pedal travel), disables the energy recovery system, and limits ESP/ABS functions.

In extreme cases, it compromises braking performance.

This is a safety-related fault.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Damaged master cylinder position sensor B inside the IPB electro-hydraulic module: The sensor is highly integrated with the IPB. A faulty internal Hall element or signal processing circuit causes abnormally high output voltage. This is the most common root cause.
  • 2Wiring harness short circuit: The sensor signal wire (typically the standard 5V reference circuit) shorts to the vehicle 12V power wire or 5V reference line, pulling the voltage high.
  • 3Connector issue: Water ingress, corrosion, or bent pins at the IPB module connector cause abnormal continuity between the signal and power terminals, commonly after driving through water or high-pressure washing.
  • 4Abnormal IPB power supply: 12V supply voltage to the IPB module rises abnormally (e.g., alternator regulator fault), causing an internal sensor supply reference shift.
  • 5Software calibration error: In rare cases, the IPB control software contains an incorrect voltage threshold calibration for Sensor B or missing calibration data, triggering a false fault (reflash the software to resolve).
What To Do
  • 1
    Safety Confirmation and Initial Inspection: Verify the brake fluid level is normal. Inspect the IPB module for physical damage and brake fluid leaks. Check the instrument panel warning light status (ABS/ESP/brake system lights). Use the VDS2000/VDS6000 diagnostic tool to read all fault codes and record freeze frame data. Verify C05CD00 is a current fault, not a history fault.
  • 2
    Data stream analysis: Access the IPB system and read the real-time voltage data of master cylinder position sensor B. Verify if the voltage remains continuously above 4.8V (normal range is typically 0.5-4.5V). Simultaneously check the voltage value of sensor A for comparison. If sensor A is normal and sensor B is abnormal, initially rule out a common power supply issue.
  • 3
    Wiring harness and connector inspection: Disconnect the IPB module low-voltage connector (usually located near the firewall). Verify the connector seal is intact and inspect the pins for water ingress corrosion (blue-green oxide) or burning marks. Use a multimeter to measure the Sensor B signal wire resistance to ground (should be infinite) and continuity to the 12V power supply (should have no continuity). Inspect the wiring harness for abrasion, damaged insulation, or contact with the vehicle body power wire.
  • 4
    Sensor and module inspection: Measure the 5V reference voltage for Sensor B at the IPB connector (4.9–5.1V). If the supply is normal but the internal output is abnormal, diagnose an internal IPB fault. If the supply voltage is too high, check the vehicle power supply system. Measure the resistance between the Sensor B signal wire and ground to verify no short circuit.
  • 5
    IPB assembly replacement: Master cylinder position sensor B integrates into the IPB and does not support separate replacement. Replace the complete IPB electro-hydraulic module assembly. Before replacement, power down the old module, disconnect the 12V battery negative terminal, and protect the brake lines to prevent brake fluid loss.
  • 6
    System matching and calibration: After installing the new IPB, connect the diagnostic tool and perform: ① Brake system bleeding procedure (execute electronic bleeding in the order: front left - front right - rear left - rear right); ② Master cylinder position sensor calibration (sensor zero-point learning); ③ Brake pedal position learning; ④ Clear all fault codes and perform a road test to confirm normal brake assist operation and that the fault code does not recur.
Real-World Cases
BYD DTC AI Analysis

BYD Seal IPB master cylinder sensor B circuit fault

✓
2022 Seal 700km Long Range variant, 23,000 km. Owner reported intermittent "Check Braking System" message on the instrument cluster, ESP warning light on, and occasional hard brake pedal. VDS scan retrieved IPB code C05CD00 (current fault); master cylinder position sensor B voltage read 5.12V (exceeding 4.8V upper limit). IPB module had no external damage and the connector showed no water ingress. Measured sensor B signal line: intermittent short to 5V reference. Replaced IPB assembly, performed sensor calibration and brake bleeding. Fault resolved. Root cause: cold solder joint on the sensor signal trace inside the IPB PCB caused intermittent short to 5V reference.
Original source ↗
BYD DTC AI Analysis

Song PLUS DM-i brake booster abnormality, DTC C05CD00

✓
2021 Song PLUS DM-i, 18,000 km. Owner reported sudden increased brake pedal travel at high speed, requiring heavy pedal pressure to brake, with multiple brake warning lights on. Towed to the shop, VDS scan of the IPB system showed DTCs C05CD00 and C05CF00 (Sensor B Performance Range). Live data showed Sensor B voltage fixed at 5.0 V, not changing with pedal movement. Inspection of the IPB connector found pin 12 (signal wire) slightly backed out and contacting adjacent pin 14 (12 V supply), pulling the signal voltage high. Repaired the pin and resecured the connector. Voltage returned to normal. Cleared the DTCs and road-tested 20 km with no fault recurrence.
Original source ↗
BYD DTC AI Analysis

Pre-delivery IPB sensor fault on new Dolphin

✓
2023 Dolphin Fashion Edition. During PDI, the instrument cluster displayed 'Brake System Fault' and the vehicle could not be delivered. Scanner read DTC C05CD00 (Master Cylinder Position Sensor B Voltage Too High). Brand new vehicle with only 15 km. Visual inspection of the IPB module and wiring harness showed no abnormalities or water ingress. IPB supply voltage measured 12.4V (normal), but Sensor B output was 5.3V. Performed IPB software upgrade (V1.2 to V1.4); fault remained. Diagnosed as production defect in IPB internal Sensor B. Replaced IPB assembly, completed calibration, cleared fault. Vehicle delivered normally. Case indicates incoming parts quality issue.
Original source ↗
Other Atto 8 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.