AU
C05CF00

DTC C05CF00 indicates the signal voltage or travel value from Master Cylinder Position Sensor B inside the IPB (Intelligent Integrated Braking System) falls outside the calibrated range (typically 0 — Seal U

Braking System

DTC C05CF00 indicates the signal voltage or travel value from Master Cylinder Position Sensor B inside the IPB (Intelligent Integrated Braking System) falls outside the calibrated range (typically 0.5-4.5V or the corresponding physical travel limit).

This sensor features a dual-redundant design (Sensors A and B) to monitor brake master cylinder piston displacement in real time, enabling brake pedal travel interpretation, precise brake force distribution, and redundant safety cross-checking.

The IPB control unit sets this fault if Sensor B experiences an open circuit, short circuit, mechanical binding, or excessive signal deviation from Sensor A.

The fault may cause abnormal brake pedal feel (soft/hard), restricted regenerative braking, ABS/ESC system deactivation, and Automatic Emergency Braking (AEB) failure.

In extreme cases, it triggers limp mode (speed-limited driving).

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Hardware fault in IPB assembly master cylinder position sensor B: damaged internal magnetoresistive element, failed Hall effect chip, or dry solder joint causing the output signal to drift or go out of range.
  • 2Wiring harness and connector issues: poor contact in Sensor B's power supply, signal, or ground circuits; oxidized or annealed terminals; water ingress or moisture in the connector causing reduced insulation resistance; or wiring harness wear causing a short circuit.
  • 3Power supply and ground fault: Unstable constant battery or ignition voltage to the IPB controller (below 11V or above 14V), or a loose ground wire bolt causing excessive contact resistance (>0.1Ω), resulting in sensor reference voltage drift.
  • 4Mechanical installation and hydraulic faults: Deformed IPB assembly mounting bracket causing coaxiality deviation between the sensor and the master cylinder piston, sticking brake master cylinder piston, or return spring failure causes the sensor to detect abnormal travel.
  • 5Software and calibration issues: IPB controller software bug, or missing or corrupted sensor calibration data causes signal interpretation errors (resolve via software update or recalibration).
What To Do
  • 1
    Safety preparation and fault confirmation: Use VDS2000 or the dedicated BYD diagnostic tool to read the complete fault codes and freeze frame data. Record the vehicle speed, pedal travel, and sensor A/B voltage values at the time of the fault; check the instrument panel warning light status and verify normal brake pedal force.
  • 2
    Visual and basic inspection: Check the IPB assembly for impact damage or brake fluid leaks; check that the brake fluid level is between MAX and MIN, and add DOT4 brake fluid if necessary; check the IPB mounting bracket tightening torque (standard 25±2Nm) and mounting surface flatness.
  • 3
    Electrical system inspection: Disconnect the IPB wiring harness connector. Measure the voltage at Pin30 (constant power) and Pin32 (ignition power); the voltage must be 12V±0.5V. Measure the resistance between ground Pin15 and Pin16 and the vehicle body ground; the resistance must be <0.1Ω. Check the sensor B power supply (usually a 5V reference voltage) and signal wire continuity.
  • 4
    Sensor signal analysis: Connect the diagnostic tool to read the data stream. Compare the real-time values of master cylinder position sensors A and B. The values must change linearly and synchronously (difference <5%). If the sensor B signal jumps, remains fixed, or falls outside the 0.5-4.5V range, confirm a sensor fault.
  • 5
    Connector specific treatment: Inspect the sensor wiring harness connector pins for oxidation, annealing, or water ingress. Clean with electronic contact cleaner, apply conductive grease (such as Stabillant 22), and verify the waterproof sealing ring is intact.
  • 6
    Calibration procedure: Clear the fault code, then perform 'Master Cylinder Position Sensor Calibration' (Sensor Calibration) and 'Zero Point Calibration'. Verify success. If calibration fails or the fault code reappears immediately, replace the IPB assembly.
  • 7
    IPB assembly replacement: If testing confirms a faulty internal sensor, replace the IPB Integrated Power Brake system assembly (verify the part number for the vehicle model, e.g., 10245087-00 for Song PLUS DM-i). Before replacement, disconnect the battery and wait 3 minutes to discharge residual high voltage.
  • 8
    Initialization after replacement: After installing the new IPB, perform the brake system bleeding procedure (use the diagnostic tool to activate motor pump bleeding and assist manually if necessary). Recalibrate the master cylinder position sensor, lateral acceleration sensor, and steering angle sensor.
  • 9
    Function check: After completing calibration, perform a road test. Test low-speed (<30km/h) and high-speed (>60km/h) braking performance. Verify ABS, ESC, and AEB functions operate normally. Observe the data stream to confirm sensor A/B signals synchronize and values are within the normal range.
Real-World Cases
BYD DTC AI Analysis

Seal DM-i: Oxidized IPB sensor pins caused abnormal signal

✓
Dashboard displayed "Check braking system", ABS/ESC fault lights illuminated, and brake pedal felt stiff. Scanned with VDS2000 and retrieved DTCs C05CF00 (Master cylinder position sensor B out of range) and C05CE00 (Sensor B voltage low). Checked IPB controller power and ground — both normal. Inspected master cylinder position sensor harness connector and found white oxidation on Pin 3 (signal) and Pin 4 (ground). Data stream showed Sensor B voltage jumping erratically between 0.2-4.8V while Sensor A varied linearly. Cleaned pins, applied conductive grease, and test drove; fault temporarily cleared but returned after 3 days. Replaced IPB assembly, performed brake bleeding and sensor calibration; fault permanently resolved.
Original source ↗
BYD DTC AI Analysis

Poor ground on Song PLUS DM-i caused Sensor B undervoltage and out-of-range condition

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Intermittent "Braking System Fault" warning on startup. Warning cleared after restart but reappeared while driving. Scanner detected C05CF00 and C05CE00 simultaneously. Brake fluid level normal, no leaks. IPB ignition supply voltage fluctuating 9–11V. Harness traced to IPB controller ground at left front longitudinal beam: retaining bolt corroded and loose, contact resistance 0.8Ω (spec <0.1Ω). Ground point cleaned to bare metal, new M8 bolt installed with anti-corrosion compound, tightened to 25Nm. Ground resistance retested at 0.03Ω. Fault codes cleared, no recurrence. No parts replaced.
BYD DTC AI Analysis

Han EV IPB mounting bracket deformation causes sensor signal mismatch

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While driving, the vehicle suddenly experienced power limitation. The dashboard displayed "Brake system fault", brake pedal travel increased, and the pedal required deep application to generate braking force. Read DTCs C05CF00 (Sensor B out of range) and C05D000 (A/B sensor mismatch). Data stream showed Sensor A at 15% piston position and Sensor B at 89%—the difference far exceeds the threshold (normal is <5%). Inspection found the IPB assembly mounting bracket had deformed slightly when the vehicle bottomed out. The IPB and brake master cylinder were misaligned, creating side force on the piston during movement. Adjusted the IPB mounting position, used feeler gauges to ensure the mounting surface gap was under 0.1mm, and tightened to the specified torque. Performed master cylinder position sensor calibration; the A/B sensor values synchronized and the fault cleared.
BYD DTC AI Analysis

Water ingress into Seal 06 DM-i sensor connector caused intermittent fault

✓
Owner reported the instrument cluster consistently displayed a brake system fault after rain or washing, clearing once dry. Inspection found the IPB mounted low with a degraded waterproof seal on the sensor harness connector, allowing water ingress. The sensor B signal line measured only 0.5 MΩ insulation resistance (specification: >10 MΩ), causing signal leakage to ground and pulling the voltage below the lower limit. Dried the connector with compressed air, replaced the waterproof seal, and applied waterproof sealant to the damaged section of the wiring harness corrugated tube. Advised the owner to avoid directing high-pressure washers at the IPB area; the fault has not recurred.
BYD DTC AI Analysis

Tang DM-p IPB software false positive causing intermittent C05CF00

✓
No actual braking fault, but the brake warning light on the dash came on intermittently with DTC C05CF00. Checked sensor wiring, power supply and ground — all normal. Data stream showed Sensor B signal within normal range but with occasional instantaneous spikes (<50ms). Consulted BYD technical bulletin: confirmed as IPB controller software V1.02 bug. Under specific road conditions (continuous bumpy roads), the signal sampling filter algorithm defect causes false positives. Upgraded IPB software to V1.05, cleared historical DTCs, performed sensor calibration, and monitored for 1 week — no recurrence.
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.