AU
C2A2000

DTC C2A2000 indicates that during pressure compensation, IPB (Intelligent Integrated Braking System) hydraulic Circuit C detects a compensation amount exceeding the internal ECU threshold (typically 15–20% above the rated compensation amount) — Atto 8

Braking System

DTC C2A2000 indicates that during pressure compensation, IPB (Intelligent Integrated Braking System) hydraulic Circuit C detects a compensation amount exceeding the internal ECU threshold (typically 15–20% above the rated compensation amount).

In the BYD One-Box brake-by-wire architecture, "Circuit C" specifically refers to the master cylinder pressure compensation circuit or the high-pressure accumulator maintenance circuit.

When the system attempts to maintain or build brake pressure, the ECU logs excessive compensation if it detects an excessively long motor-driven hydraulic pump compensation time, an abnormally high compensation frequency, or a compensation pressure gradient outside the calibrated range.

This typically indicates an internal hydraulic circuit leak (poor solenoid valve sealing), abnormal brake fluid compressibility (air ingress or contamination), pressure sensor feedback drift, or reduced hydraulic pump volumetric efficiency.

This fault triggers the braking system to enter a degraded mode (backup braking mode), limits ABS/ESC functions, and may increase brake pedal travel and braking distance.

This is a Level 2 fault affecting driving safety.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Aged, deformed, or binding internal solenoid valve seal in the IPB electro-hydraulic control module causes high-pressure fluid to leak internally from circuit C to the low-pressure side, forcing the system to continuously compensate for pressure.
  • 2Severely contaminated brake fluid (water content >3% or containing impurities) causes valve body sealing surface corrosion and poor lubrication, increasing internal leakage.
  • 3Signal drift or intermittent open circuit at the Circuit C pressure sensor (master cylinder pressure sensor) sends a false low-pressure signal to the ECU, causing the system to misjudge and continuously increase pressure to compensate.
  • 4Wear in the plunger pump or DC motor inside the IPB assembly reduces volumetric efficiency, preventing normal pressure buildup and triggering continuous compensation mode.
  • 5A minor external leak in the hydraulic lines or wheel cylinders (e.g., loose fittings or micro-cracks in hoses) causes a continuous pressure drop in circuit C, triggering compensation.
What To Do
  • 1
    Use VDS2000 or a dedicated BYD diagnostic tool to read the complete DTC list. Check for accompanying C2A1700 (hydraulic circuit overcompensation), C2A1F00 (circuit A overcompensation), or pressure sensor fault codes, and analyze the fault correlation.
  • 2
    Check that the brake fluid reservoir level is between MAX and MIN. Use a brake fluid tester to check the brake fluid color and moisture content. If the moisture content is >2.5% or the fluid is dark and cloudy, completely replace it with DOT4 standard brake fluid.
  • 3
    Visually inspect the IPB module body, high-pressure lines, and all four brake wheel cylinders for signs of leakage. Focus on dampness or fluid traces at the IPB-to-line connections.
  • 4
    Connect the diagnostic tool and enter 'IPB System - Data Stream'. Monitor parameters such as 'Circuit C Compensation Percentage', 'Master Cylinder Pressure', and 'Motor Current'. Normal static compensation should be <5%. If the value remains >15% or pressure fluctuates abnormally, confirm the fault exists.
  • 5
    Perform the IPB system bleeding procedure (use a diagnostic tool to trigger the hydraulic pump for automatic bleeding; traditional manual bleeding cannot completely purge internal air). After completion, clear the DTC and perform a road test.
  • 6
    If the fault persists, disconnect the IPB wiring harness connector. Measure the pressure sensor supply voltage (should be 5V±0.25V), ground resistance (<1Ω), and signal voltage (0.5-4.5V corresponding to the pressure range). Check the wiring harness for poor connections.
  • 7
    Replace the IPB electro-hydraulic control module assembly (select the part number based on vehicle configuration, e.g., 6A-3505010 series for Song PLUS DM-i). After replacement, perform the 'IPB Assembly Replacement' online configuration and coding matching, and bleed all four brakes.
Real-World Cases
BYD DTC AI Analysis

Song Plus DM-i IPB internal leak caused Circuit C overcompensation

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Vehicle: 2021 Song PLUS DM-i, 32,000 km. Customer reported intermittent ABS/ESC warning lights and occasional brake pedal kick-back during braking. VDS scan showed DTCs C2A2000 and C2A1F00 stored. Live data showed Circuit C compensation fluctuating between 18-25%, with master cylinder pressure dropping at 0.3 MPa/s during the hold phase. Disassembled the IPB module and found the master cylinder valve seals aged and deformed, allowing high-pressure fluid to leak back to the reservoir through the valve spool clearance. Replaced the IPB assembly. Compensation returned to under 3% and the fault cleared.
BYD DTC AI Analysis

False sensor readings caused by brake fluid contamination

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Vehicle: 2022 Song Pro DM-i, 18,000 km. Fault appeared one week after brake fluid replacement at a non-authorised workshop. Scanner showed DTC C2A2000; no other pressure fault codes. Live data showed Circuit C pressure sensor signal jumping erratically between 0.8–1.2 V (normal static reading is ~1.5 V). Inspection found brake fluid milky and contaminated (mixed with water or other liquid), corroding the pressure sensor diaphragm. Thoroughly flushed the brake lines, replaced the IPB module and filled with fresh brake fluid. Fault resolved.
BYD DTC AI Analysis

Intermittent fault due to poor contact in IPB wiring harness

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Vehicle: 2021 BYD Song PLUS DM-i, 45,000 km. Intermittent fault code C2A2000, easily triggered on rough roads. Inspected the IPB module wiring harness connector near the bulkhead and found pin 12 of the X1 connector (Circuit C pressure sensor signal line) backed out and corroded. Contact resistance spiked from 0.5Ω to over 15Ω under vibration, causing the ECU to receive abnormal voltage and falsely detect insufficient pressure. Repaired the harness connector and re-crimped the terminals; fault cleared.
BYD DTC AI Analysis

Software calibration abnormality caused excessive compensation

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Vehicle: 2021 Song PLUS DM-i. C2A2000 found during new-vehicle PDI. No abnormal brake feel, no external leaks, brake fluid level normal. Data stream showed Circuit C compensation at 16% (threshold 15%). Technical bulletins indicated overly sensitive IPB software calibration for this batch. Updated IPB ECU software (V1.02 → V1.05) and reran Linear Valve Calibration. Compensation dropped to 8%; DTC cleared, no recurrence.
BYD DTC AI Analysis

Circuit pressure retention fault caused by minor leak in right rear wheel cylinder

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Vehicle: 2022 Song MAX DM-i, 51,000 km. Instrument cluster displayed "Brake System Fault," DTC C2A2000. Inspection found slight leakage from the right rear wheel cylinder and minor oil staining on the back of the brake disc. Circuit C maintains master cylinder pressure; the leaking wheel cylinder caused continuous compensation. Replaced the right rear wheel cylinder repair kit, cleaned the brake disc, topped up brake fluid and bled the system. Compensation returned to normal. This case shows Circuit C faults don't always come from the IPB unit—check sealing throughout the entire hydraulic circuit.
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.