AU
B17A700

This DTC indicates the Airbag Control Unit (ACU) recorded a rear collision event — Seal U

Safety System

This DTC indicates the Airbag Control Unit (ACU) recorded a rear collision event.

The system sets this code when the Rear Impact Sensor detects a deceleration threshold trigger in a specific direction, or the accelerometer integrated within the SRS control unit identifies a G-value change consistent with a rear collision.

This event-recording DTC may accompany seat belt pretensioner activation or an airbag deployment command.

Upon storing this code, the system illuminates the airbag fault warning lamp, may disable specific airbag functions to prevent unintended secondary deployment, and records key collision data in the freeze frame (such as vehicle speed, seat occupancy status, and seat belt buckle switch status) for accident analysis.

If no actual collision occurred, the sensor or control unit has an electrical fault or signal interference.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1An actual rear-end collision caused the rear impact sensor (located on the rear panel or C-pillar) to reach its trigger threshold.
  • 2Deformed or loose rear crash sensor mounting bracket, or damaged piezoelectric element inside the sensor body, causing a false alarm.
  • 3Sensor wiring harness short to ground, short to power, or electromagnetic interference affecting the signal circuit generates a false collision signal.
  • 4SRS control unit internal acceleration sensor fault, or software algorithm misjudgment identifying normal bumps as a collision.
  • 5A severe drop in vehicle 12V battery voltage or poor system ground causes abnormal control unit signal sampling.
What To Do
  • 1
    Use VDS2000 or the latest diagnostic tool to read the complete SRS system fault codes and freeze frame data. Confirm the specific collision detection sensor location, trigger time, and vehicle status parameters.
  • 2
    Visually inspect the rear of the vehicle (rear bumper, rear body panel, tail lamp housing) for signs of collision, repair, or water ingress.
  • 3
    Remove the rear bumper. Check the rear crash sensor physical mounting condition, connector security, and wiring harness insulation for damage. Measure the sensor power supply (usually a 5V reference voltage) and signal output.
  • 4
    Use an oscilloscope to monitor the sensor signal wire waveform. Lightly tap the sensor mounting base to simulate slight vibration and check for abnormal voltage spikes.
  • 5
    Check the SRS control unit appearance, mounting torque, and power/ground points (focusing on instrument panel frame ground point G301). Perform control unit self-learning calibration if necessary.
  • 6
    If confirming no actual collision history and the hardware is normal, update the SRS control unit software to the latest version, clear the fault code, and perform a road test to verify. If the fault recurs, replace the rear crash sensor or ACU assembly.
  • 7
    After completing the repair, use the dedicated collision simulator to verify sensor function and confirm the system has returned to normal monitoring status.
Real-World Cases
BYD DTC AI Analysis

BYD Qin PLUS DM-i compressor insulation fault

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Symptoms: At ~20,000 km, AC suddenly stopped cooling after 10-15 minutes of operation. No warning lights on the instrument panel, but the AC panel flashed. VDS retrieved DTC B17A700 (electric compressor overcurrent protection). Temporarily recovered after shutdown and restart, then failed again. Diagnosis: 1. Read AC controller data stream: compressor target speed 3000rpm, actual 0rpm, drive current abnormally high (>15A, normal 4-8A). 2. Measured electric compressor high-voltage harness insulation resistance to ground: only 0.5MΩ (standard >20MΩ). Determined internal motor insulation failure. 3. Refrigerant pressure normal. Ruled out mechanical seizure causing overcurrent. Resolution: Replaced electric AC compressor assembly (Part No.: 8103020-XX); replaced receiver drier and thoroughly flushed AC lines (prevents metal debris damaging new compressor); evacuated and recharged with 450g±25g R134a refrigerant; flashed AC controller to latest program version.
Original source ↗
BYD DTC AI Analysis

BYD Song Pro DM: Loose connection in AC compressor controller wiring

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Symptoms: In hot summer weather, the AC cooling cuts out intermittently—the fault comes and goes. DTC B17A700 triggers sporadically, sometimes accompanied by B17A600 (compressor drive circuit fault). Diagnosis: 1. When the fault occurred, measured voltage at the compressor controller low-voltage connector: 12V supply read normal, but CAN-H and CAN-L showed abnormal fluctuations. 2. Removed the right front wheel arch liner to inspect the compressor controller connector (located at the right front of the engine bay). Found slight green oxidation on the internal pins and loose mating between the male and female halves. 3. While checking harness continuity, measured 12Ω contact resistance on the HVIL (High Voltage Interlock Loop) pin—specification calls for less than 1Ω. This high resistance caused the controller to false-trigger a high-voltage fault and enter protection mode. Repair: Extracted the compressor controller connector pins with a terminal removal tool and cleaned off the oxidation using anhydrous alcohol. Adjusted the terminal spring tension to ensure tight contact, then applied conductive grease (electrical contact lubricant) to prevent future oxidation. Reconnected the plug, wrapped the connection with waterproof tape, and secured the harness with zip ties to prevent loosening from road vibration.
Original source ↗
BYD DTC AI Analysis

BYD Han EV Heat Pump System Software Logic Fault

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**Symptoms:** 2021 Han EV AWD High Performance. When using the heat pump for heating in winter, the vehicle frequently sets code B17A700 and heating performance drops significantly, though summer cooling works normally. The dealership replaced the compressor multiple times without resolving the issue. **Diagnosis:** 1. Comparing live data with a normal vehicle revealed that at ambient temperatures around 0°C, the compressor draws 18A inrush current at startup, triggering overcurrent protection; 2. Inspection confirmed the heat pump system's four-way valve and electronic expansion valve operate normally, with correct refrigerant charge; 3. Manufacturer technical support confirmed a software bug in the AC controller for this production batch: the compressor startup ramp current control logic is flawed in low temperatures, causing excessive startup current. **Solution:** No hardware replacement needed. Upgrade the AC controller software to V2.8.1 or higher. After upgrading, recalibrate the compressor minimum startup current and ramp-up rate. Clear all fault codes and run a 48-hour low-temperature test to confirm the repair.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM: Poor condenser cooling causing overcurrent

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Symptoms: Tang DM A/C suddenly shut down during long-distance highway driving or after fast charging, with the instrument cluster showing "Air Conditioning System Fault." Scanned DTC B17A700; compressor temperature sensor read over 120°C. Diagnosis: 1. Condenser surface severely blocked by willow fluff and dust, with over 40% of cooling fins collapsed. 2. Compressor was drawing 12A continuously at 35°C ambient (normal: 8-10A), causing overheating under sustained high load. 3. Electric fan low-speed operation normal, but high-speed only reached 60% of normal RPM, further weakening cooling capacity. Fix: Thoroughly cleaned condenser fins: blew out with high-pressure air, flushed with A/C cleaner, then rinsed with clean water (protecting high-voltage components). Replaced the electric fan assembly (worn high-speed carbon brushes). Checked refrigerant purity, found trace air contamination; evacuated the system and recharged with the standard refrigerant quantity. Advised owner to clean the condenser front filter every quarter.
Original source ↗
BYD DTC AI Analysis

BYD e5 taxi: Compressor mechanical seizure

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Symptoms: e5 taxi (180,000 km). Heard a 'click' the instant the AC turned on; compressor then stopped working. Fault code B17A700 present and will not clear. Compressor had been noisy for roughly six months prior. Diagnosis: 1. Disconnected the compressor HV harness and rotated the clutch plate by hand. Extreme resistance and obvious metallic grinding noise. 2. Disassembled the compressor: scroll set severely worn, heavy metal debris inside, motor winding insulation damaged. 3. Cause: long-term infrequent AC use (driver typically opened windows to save power), resulting in poor internal lubrication, dry friction, and eventual seizure of the scroll set. Repair: Replaced the electric compressor (used a remanufactured part to reduce cost). Fully flushed the AC system: ran a dedicated flushing machine through the evaporator, condenser and lines, replaced the expansion valve and desiccant (prevents metal debris from the old system damaging the new compressor). Advised the driver to run the AC at least 15 minutes weekly to maintain compressor lubrication.
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.