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 — Atto 8

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 — Atto 8
  • 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 state.
  • 2Sensor wiring harness short to power: 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, resulting in abnormally high actual pressure.
  • 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: An 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 all fault codes. Check the 'actual master cylinder pressure' in the freeze frame data (normal: 0-1 MPa during pedal free travel; typically <18 MPa when fully depressed). A reading >25 MPa or 4.9 V without pedal application indicates a sensor or wiring fault. A stiff 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; Song PLUS DM-i uses 48-pin). Measure the resistance between the pressure sensor signal wire (usually Pin 23 or Pin 24; check the wiring diagram for the specific pin) and body ground (must be >1MΩ). Measure the resistance between the signal wire and the 5V reference wire (Pin X) to confirm no short circuit exists. Test the signal wire for a short circuit to the power supply (Pin 1, constant power). Inspect the connector for water ingress, corrosion, or 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 end. If the data stream shows normal pressure at 3V input, but sets C05D309 at 4.8V input, the ECU detection logic is normal and the fault lies in the sensor or wiring harness. If the data stream still shows high pressure at 3V input, replace the IPB-ECU.
  • 4
    Hydraulic system pressure relief and inspection: If you suspect the actual pressure is too high, carefully loosen the brake caliper bleed screw to relieve pressure (wear protective equipment) and 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 the following: 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 regenerative braking 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 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.