AU
C2A1700

DTC C2A1700 indicates the pressure compensation value in the IPB (Intelligent Power Brake) internal hydraulic circuit exceeds the normal ECU-calibrated threshold — Seal U

Braking System

DTC C2A1700 indicates the pressure compensation value in the IPB (Intelligent Power Brake) internal hydraulic circuit exceeds the normal ECU-calibrated threshold.

Specifically, during pressure holding or build-up in the L1 hydraulic circuit (master cylinder primary circuit or a specific wheel cylinder circuit), the compensation frequency or amount the system requires to maintain target pressure constantly exceeds normal values.

Delayed pressure build-up or pressure fluctuations usually accompany this condition.

The ECU triggers this fault upon detecting abnormal hydraulic system compressibility (e.g., air ingress) or an internal leak, which forces the system to overwork to maintain pressure.

This represents a self-check anomaly within the IPB electro-hydraulic control module.

A persistent fault can cause abnormal brake pedal travel, restrict ABS/ESC functions, or disable Automatic Emergency Braking (AEB), posing a driving safety hazard.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Air in the brake hydraulic system: Incomplete bleeding after replacing brake fluid or repairing brake lines, or poorly sealed line connections drawing in air, increases hydraulic compressibility and requires the IPB motor to frequently compensate for pressure.
  • 2IPB electro-hydraulic module internal pressure sensor fault: L1 circuit pressure sensor signal drifts or fails, sending an incorrect pressure value to the ECU and causing the system to falsely detect an overcompensation condition.
  • 3Brake fluid contamination or deterioration: Excessive water content (>3%) or impurities in the brake fluid cause valve body binding and seal swelling, impairing hydraulic circuit sealing.
  • 4Damaged internal IPB hydraulic valve body or sealing ring: A sticking L1 circuit solenoid valve or check valve, or an aged and cracked sealing ring, causes pressure leakage during the pressure hold phase, triggering continuous compensation.
  • 5Brake line physical leak: Microscopic leak in the L1 circuit lines, hoses, or wheel cylinder causes the system to continuously detect a low-pressure condition and attempt compensation.
What To Do
  • 1
    Use the VDS2000/Launch X431 diagnostic tool to read all fault codes, confirm if C2A1700 is active, and read the freeze frame data to record the vehicle speed, pressure value, and pedal travel when the fault occurred.
  • 2
    Check if the brake fluid reservoir level is between MIN and MAX. Check the brake fluid color and moisture content (using a brake fluid moisture tester). If the moisture content is >2.5% or the color is cloudy, replace with DOT4 low-viscosity brake fluid.
  • 3
    Visually inspect the IPB module and L1 circuit lines (usually the left front wheel or master cylinder first circuit) for obvious leaks, bulging, or loose connections. Focus on the IPB master cylinder high-pressure outlet fitting.
  • 4
    Perform the IPB system bleeding procedure (use the diagnostic tool to activate bleeding mode): conduct conventional bleeding in the sequence of right rear, left rear, right front, and left front. Next, perform IPB internal bleeding to ensure the L1 circuit is free of air.
  • 5
    After bleeding, clear the fault code and perform the IPB function test: cycle the ignition switch ON/OFF three times to complete the IPB self-check and check if the code returns. Conduct a road test with multiple hard braking applications to activate the ABS and observe if the hydraulic circuit pressure builds normally.
  • 6
    If the fault persists, use the diagnostic tool to read the L1 circuit real-time pressure data and compare the actual pressure to the target pressure. If the deviation is >10 bar and the motor runs continuously, the IPB internal valve body or sensor is faulty. Replace the IPB electro-hydraulic module assembly with ECU.
  • 7
    After replacing the IPB module, perform the following: coding, longitudinal acceleration sensor calibration, pedal position learning, and a four-wheel speed sensor signal check. Finally, conduct a complete road test for verification.
Real-World Cases
BYD DTC AI Analysis

C2A1700 fault code found during PDI of new BYD Song Plus DM-i

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Vehicle: 2021 Song Plus DM-i, pre-delivery PDI inspection. Symptom: No warning lights on the dashboard, but the diagnostic tool logged historical DTC C2A1700. Diagnosis: Brake fluid level checked normal; no leaks found. Tiny air bubbles detected during IPB system self-testing in transit triggered the code. Repair: Ran the IPB dedicated bleed program (activated bleed pump mode via diagnostic tool), performed three bleed cycles on the L1 circuit, cleared the DTC, and road-tested 10 km. Fault did not recur. Follow-up: Tracked for one month post-delivery. No recurrence.
BYD DTC AI Analysis

Seal 6 DM-i: Warning light on after brake caliper replacement

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Vehicle: Seal 6 DM-i, 23,000 km. Repair history: Left front collision, replaced brake hose and caliper. Fault: After replacement, ABS/ESC warning lights stayed on. DTC C2A1700 (hydraulic circuit L1 overcompensation). Diagnosis: Found left front brake line disconnected during bleeding. Suspected air entered the IPB internal L1 circuit. Conventional bleeding did not clear the fault. Resolution: Connected a diagnostic scan tool and accessed the IPB special functions. Performed "Internal Bleeding" and "Pressure Sensor Zero Calibration." Cleared the DTCs, then completed multiple emergency stops from 20 km/h to 0 km/h on a flat, straight road. System returned to normal.
BYD DTC AI Analysis

IPB sensor drift caused intermittent fault

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Vehicle: Song Pro DM-i, 48,000 km. Symptom: Intermittent "Braking System Fault" dashboard warning; DTC C2A1700 appearing sporadically, especially on cold starts. Diagnosis: Brake fluid showed no degradation; lines had no leaks; multiple bleeds ineffective. Live data revealed L1 circuit pressure sensor signal drifting 0.8 bar at rest (normal: 0±0.2 bar). Resolution: Confirmed internal IPB pressure sensor failure. Replaced IPB electro-hydraulic module assembly (with ECU), performed module coding and sensor calibration. Fault eliminated.
BYD DTC AI Analysis

Expired, contaminated brake fluid caused compensation malfunction

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Vehicle: Qin PLUS DM-i, 3 years without brake fluid change. Symptoms: ABS warning light on, brake pedal travel increased. Fault codes C2A1700 and C2A2000. Diagnosis: Brake fluid moisture content tested at 4.2%, dark brown. Disassembled IPB outlet port and found gum deposits at the valve orifice. Degraded brake fluid stuck the L1 circuit valve and degraded the seals. Solution: Replaced brake fluid, flushed lines with dedicated cleaning fluid, and cleaned the IPB valve body using a non-dismantle cleaning machine. Cleared fault codes and pedal travel returned to normal.
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.