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C05D300

C05D300 is a specific fault code for the BYD IPB (Intelligent Power Brake) integrated braking system — Seal U

Braking System

C05D300 is a specific fault code for the BYD IPB (Intelligent Power Brake) integrated braking system.

It indicates the actual hydraulic pressure detected by the master cylinder pressure sensor exceeds the theoretical expected value the ECU calculates using the brake pedal position sensor (master cylinder stroke sensor A/B).

This fault represents a logical mismatch between the pressure signal and the pedal stroke signal.

Possible causes include: 1) Pressure sensor drift or damage causing a falsely high signal; 2) Incomplete brake master cylinder piston return generating residual pressure; 3) Hydraulic circuit abnormalities (blockage or internal leakage) causing abnormal pressure transfer; 4) Incorrect brake pedal position sensor signals causing the ECU to miscalculate the expected value; 5) IPB control module software algorithm errors.

This fault triggers the brake system degraded mode.

It may restrict ABS/ESP functions, cause a hard brake pedal or abnormal pedal travel, and severely compromise driving safety.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Signal drift, damage, or mounting misalignment of the brake master cylinder position sensor (especially Sensor B), causing abnormal feedback voltage (exceeding the 4.5V standard range).
  • 2Sticking brake master cylinder piston, aged and swollen seal ring, or failed return spring prevents complete release of residual pressure after pedal release (static pressure > 0.3 MPa).
  • 3A sticking hydraulic valve body inside the IPB electro-hydraulic module or a partially blocked brake line causes abnormal pressure build-up and slow pressure release.
  • 4Loose connection, oxidation, water ingress, or poor ground at the sensor wiring harness connector (including key terminals such as pin 14 of the IPB module 32-pin connector), causing intermittent signal voltage fluctuations.
  • 5IPB control module software bug (early versions such as V1.02) or lost master cylinder position sensor calibration data, causing an incorrect pressure-to-stroke mapping.
What To Do
  • 1
    Use the BYD VDS diagnostic tool to access the IPB system and read the complete fault code list (check specifically for accompanying related fault codes such as C05D200, C05CF00, and C055E00). Record the pressure and pedal travel values from the freeze frame data.
  • 2
    Enter data stream mode and monitor the 'master cylinder pressure sensor', 'master cylinder position sensor A', and 'master cylinder position sensor B' values in real time: at rest, the pressure should be 0 MPa and the sensor voltage 0.5-4.5V; when applying the pedal, both sensor signals should change synchronously and linearly, and the pressure-to-travel ratio should remain within the standard curve range.
  • 3
    Inspect the brake pedal mechanism: confirm the pedal does not bind and returns smoothly; check the clearance between the vacuum booster and the brake master cylinder pushrod; remove and inspect the master cylinder piston for rust and the sealing ring for swelling or deformation; replace the master cylinder assembly if necessary.
  • 4
    Check electrical connections: Disconnect the IPB module 32-pin connector; check pin 14 (sensor B signal), the power pin, and the ground pin for poor contact, backed-out terminals, or corrosion; measure wiring harness resistance (must be <1Ω) and insulation; check the ground point G06 connection.
  • 5
    Perform hydraulic system inspection: Use the diagnostic tool to perform a hydraulic circuit test to check the pressure build-up rate and pressure holding capability of each wheel cylinder (pressure drop within 10 seconds must be <0.5 MPa). Visually inspect brake hoses and wheel cylinders for leaks. Replace brake fluid and thoroughly bleed the system if necessary.
  • 6
    Software procedure: Check the IPB control module software version. If the software is an earlier version (such as V1.02), upgrade to the latest version (such as V1.08 or higher). After replacing hardware, execute the 'Master Cylinder Position Sensor Calibration' procedure. Clear the fault code, then road test the vehicle for 20 km to confirm.
  • 7
    Verify repair: Repeatedly press the brake pedal. Confirm the pressure value in the data stream matches the pedal travel with no abnormal residual pressure. Confirm the instrument cluster displays no warning lights and the ABS/ESC self-check is normal.
Real-World Cases
BYD DTC AI Analysis

Sticking brake master cylinder piston causing excessive residual pressure

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Symptoms: Song PLUS DM-i instrument cluster displayed brake system warning light with DTC C05D300. Brake pedal was stiff with slow return, vacuum assist reduced. Diagnosis: Live data showed 0.3MPa residual master cylinder pressure with pedal fully released (specification: 0MPa). Master cylinder position sensor B signal voltage measured 4.8V at rest, exceeding upper limit. Disassembly revealed slight sticking of the master cylinder piston. Solution: Replaced brake master cylinder assembly (including position sensor), bled IPB system, calibrated master cylinder position sensor using diagnostic scan tool, cleared fault codes. Completed 20km test drive with no recurrence.
Original source ↗
BYD DTC AI Analysis

Loose connection in the master cylinder position sensor wiring harness caused abnormal signals.

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Symptoms: Qin PLUS DM-i at 30,000 km, dashboard displaying 'Check Brake System' warning, DTCs C05D200 and C05D300 stored simultaneously, longer initial brake pedal travel. Diagnosis: Live data showed Sensor A signal normal when depressing the pedal, but Sensor B signal intermittently spiking to maximum; found loose connection at IPB module 32-pin connector Pin 14 (Sensor B signal line), resistance fluctuating 0.5-5Ω; UBVR power supply circuit (PIN14) had intermittent open circuit. Resolution: Repaired and re-crimped the harness terminal, sanded and tightened earth point G06, replaced brake master cylinder position sensor B, performed IPB system initialisation and calibration.
Original source ↗
BYD DTC AI Analysis

Brake hydraulic circuit leak with abnormal pressure

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Symptoms: Song Pro DM at 50,000 km stored DTCs C05D300 and C055E00 (brake hydraulic circuit A leak). Pedal travel noticeably longer; needed deep pedal application to generate braking force. Diagnosis: Visual inspection showed no external leaks, but the IPB hydraulic circuit test revealed slow pressure build-up at the left front wheel cylinder. The pressure hold test lost 2 MPa in 10 seconds (over limit). On the lift, the left front brake hose showed micro-cracks, and the master cylinder seal was leaking internally from age. Solution: Replaced the left front brake hose and master cylinder seal kit, completely replaced the brake fluid and bled the system, cleared the fault codes and ran the IPB system self-test.
Original source ↗
BYD DTC AI Analysis

Software bug in IPB module caused false positive

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Symptoms: BYD Dolphin, 3000 km, no collision history. Dashboard intermittently reported brake fault C05D300, but braking performance was normal. The fault code cleared itself but recurred. Diagnosis: Intermittent historical fault. Live data showed brake pedal position sensor and pressure sensor values were normal. The IPB module had early software version V1.02 with a known bug causing incorrect pressure threshold judgment. Solution: Upgraded the IPB control module software to V1.08 and ran system calibration and self-learn procedures. Monitored for one month; the fault did not recur.
Original source ↗
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.