AU
C05D309

This DTC indicates the master cylinder pressure sensor built into the IPB (Intelligent Power Brake) electro-hydraulic module detects a pressure signal exceeding the normal upper threshold set by the ECU (typically >25 MPa or signal voltage >4 — Seal U

Braking System

This DTC indicates the master cylinder pressure sensor built into the IPB (Intelligent Power Brake) electro-hydraulic module detects a pressure signal exceeding the normal upper threshold set by the ECU (typically >25 MPa or signal voltage >4.8 V).

In BYD e-Platform 3.0 and DM-i models, the IPB integrates conventional ESP, ABS, and i-BOOSTER functions, and uses a motor-driven plunger pump to build pressure.

The pressure sensor monitors master cylinder pressure in real time for brake force distribution, regenerative braking intensity adjustment, and active braking control.

When the sensor signal remains above the calibrated value beyond the set time (typically 200-500 ms), the ECU logs a "pressure too high" fault.

This fault restricts ABS/ESP functions, disables Automatic Emergency Braking (AEB), and interrupts regenerative braking.

In extreme cases, it triggers a "brake system fault" warning and limits vehicle speed.

Determine whether the cause is actual excessive hydraulic pressure (such as a blocked brake line) or a false high-pressure signal caused by sensor drift or a short circuit.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Pressure sensor internal short circuit or signal drift: An aging or damaged pressure-sensitive element outputs a fixed high-voltage signal (close to the 5V reference voltage), causing the ECU to falsely detect a high-pressure condition.
  • 2Sensor wiring harness short to power supply: Water ingress, wear, or pin misalignment at the IPB harness connector (usually near the firewall) shorts the pressure sensor signal wire (Pin 2 or Pin 3, depending on vehicle model) to the 12V power supply or 5V reference voltage.
  • 3IPB hydraulic circuit mechanical fault: blocked brake master cylinder return port, high-pressure valve stuck in the normally open position, or severely contaminated and crystallized brake fluid preventing pressure release and causing actual pressure to rise abnormally.
  • 4IPB-ECU internal ADC circuit fault: Abnormal ECU analog-to-digital converter reference voltage or damaged sampling circuit causes the ECU to misread normal sensor signals as out-of-range values.
  • 5Software calibration error: IPB control software version defect or failure to perform 'pressure sensor zero-point calibration' after replacing the IPB assembly shifts the reference value and triggers a false fault.
What To Do
  • 1
    Fault confirmation and freeze frame analysis: Use VDS 4.0 or 5.0 to read the complete fault codes and check the 'actual master cylinder pressure' in the freeze frame data (0-1 MPa indicates normal pedal free travel; firm pedal application usually reads <18 MPa). A reading >25 MPa or 4.9 V without pedal application indicates a sensor/wiring fault. A hard pedal with high pressure indicates a hydraulic fault. Check for related fault codes such as C05D300 and C05D500.
  • 2
    Circuit continuity and insulation test: Disconnect the IPB assembly connector (48-pin or 32-pin; 48-pin for Song PLUS DM-i). Measure the resistance between the pressure sensor signal wire (usually Pin 23 or Pin 24; refer to the wiring diagram) and body ground (should be >1 MΩ), and between the signal wire and the 5V reference wire (Pin X) (must not be shorted). Test the signal wire for a short circuit to the power supply (Pin 1, constant power). Inspect the connector for water ingress, corrosion, and backed-out pins.
  • 3
    Sensor signal simulation test: Use a variable resistor to simulate the pressure sensor (0.5-4.5V range) and connect it to the signal wire at the ECU terminal. If applying 3V shows normal pressure in the data stream, but applying 4.8V triggers C05D309, the ECU detection logic is normal; the fault lies in the sensor or wiring harness. If applying 3V still shows high pressure, replace the IPB-ECU.
  • 4
    Hydraulic system pressure relief and inspection: If suspecting the actual pressure is too high, carefully loosen the brake caliper bleed screw to relieve pressure (wear protective equipment). Observe if the pressure returns to zero. Inspect the brake fluid for cloudiness or metal shavings (indicating pump body wear). Execute the IPB bleeding procedure (VDS Special Functions → IPB Bleeding) and observe if the pressure builds and releases normally.
  • 5
    IPB assembly replacement and calibration: If diagnostics confirm an internal sensor or ECU fault, replace the IPB electro-hydraulic brake assembly (part numbers vary by model; Song PLUS DM-i uses the 6A-3555100 series). After replacement, perform: ① Brake system bleeding; ② Pressure sensor zero-point calibration (VDS → IPB System → Special Functions → Sensor Calibration); ③ Yaw rate sensor calibration; ④ Road test to verify energy recovery and ABS functions.
Real-World Cases
BYD DTC AI Analysis

Song PLUS DM-i: C05D309 and multiple system faults after water wading

✓
After wading through water, the dashboard displayed "Braking System Fault" and "ESP Fault". The VDS read multiple pressure-related DTCs including C05D309 and C05D500. Inspection found water in the IPB wiring harness connector (located at the left front fender liner). Pin 23 (pressure sensor signal) and Pin 1 (12V constant power) were shorted (0.8Ω). Cleaned the connector corrosion, applied conductive paste, and cleared the fault. Check wiring harness waterproof boot sealing on this vehicle batch.
BYD DTC AI Analysis

Replaced IPB without calibration; fault recurred.

✓
IPB assembly replaced for hard brake pedal. After 50 km, C05D309 returned. Inspection found the technician performed only conventional brake bleeding without the 'pressure sensor zero-point calibration'. Used VDS to run Special Functions → IPB System → Sensor Calibration (ignition ON, brake pedal released, follow prompts for 3-minute self-learning). Post-calibration live data showed master cylinder pressure at 0.1 MPa (standard range ±0.2 MPa). Fault cleared.
BYD DTC AI Analysis

Brake fluid crystallisation caused false pressure sensor readings

✓
2021 Song PLUS DM-i, 80,000 km. Brake fluid overdue for replacement. Intermittent DTC C05D309 with no braking force in the first third of pedal travel. Disassembled the IPB and found white crystals inside the valve body and pressure sensor chambers (additives precipitated after brake fluid absorbed moisture). Replaced the IPB assembly, thoroughly flushed the lines, and filled with DOT4 low-viscosity brake fluid (ISO 4925 Class 6). Fault resolved. For DM-i models, replace brake fluid every 2 years or 40,000 km.
BYD DTC AI Analysis

False alarm caused by software defect (OTA fix case)

✓
Multiple 2021 Song PLUS DM-i vehicles intermittently throw C05D309 on cold starts below 5°C; the code clears after warm-up. Freeze frame shows the pressure reading right at the threshold (24.8 MPa vs. 25 MPa). Manufacturer technical support confirmed this is an ADC sampling filter algorithm defect in IPB control software V1.2 at low temperatures. OTA update to V1.3 (optimizes low-temperature sampling logic) or dealer reflash of the IPB ECU software resolves the issue.
BYD DTC AI Analysis

Pressure sensor internal drift fault

✓
With the vehicle stationary, DTC C05D309 set repeatedly. Live data showed master cylinder pressure locked at 26.5 MPa (full-scale value) regardless of pedal position. Measured the pressure sensor signal voltage at the IPB connector: 4.95 V (normal at idle: 0.5–1.2 V); 5 V reference and ground were normal. Disconnected the sensor plug and the signal voltage dropped to zero, confirming no short in the wiring. Replaced the IPB electro-hydraulic module—the pressure sensor is integral to the assembly and not available separately. Fault resolved.
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.