AU
B17A300

In the BYD SRS system, DTC B17A300 carries two technical indications: 1) The surface description, "seat belt pretensioner collision", indicates the system detects a trigger signal or ignition circuit fault in the driver or passenger seat belt pretensioner (Pyrotechnic Pretensioner); 2) The underlying technical definition, "SRS CAN Signal Abnormal", means the airbag control unit (ACU) loses communication with the vehicle CAN network (typically the powertrain or comfort network) or detects a data checksum error — Seal U

Safety System

In the BYD SRS system, DTC B17A300 carries two technical indications: 1) The surface description, "seat belt pretensioner collision", indicates the system detects a trigger signal or ignition circuit fault in the driver or passenger seat belt pretensioner (Pyrotechnic Pretensioner); 2) The underlying technical definition, "SRS CAN Signal Abnormal", means the airbag control unit (ACU) loses communication with the vehicle CAN network (typically the powertrain or comfort network) or detects a data checksum error.

This fault forces the airbag system into a degraded mode, potentially preventing airbag and pretensioner deployment during a collision.

It also triggers a continuous airbag warning light on the instrument cluster.

In models such as the Qin PRO, this fault frequently accompanies B17A200 (collision record locked) or B17A400 (hardwired signal abnormal), indicating the vehicle experienced a collision or the CAN bus physical layer has an intermittent fault.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1CAN bus physical layer fault: Short circuit between CAN-H and CAN-L, short to power/ground, or terminal resistor (120Ω ±2%) drift causing signal reflection. Typically occurs at dashboard wiring harness junctions or due to floor wiring harness oxidation after water ingress.
  • 2Airbag Control Unit (SRS ECU) power supply fault: blown constant power (B+) circuit fuse, lost ignition switch signal (IGN), or loose ground points G301/G302, causing the ACU to lose power during the communication cycle.
  • 3Seat belt pretensioner ignition circuit fault: Water ingress and oxidation at the pretensioner connector (usually located below the B-pillar) cause abnormal resistance (normal: 2.0-2.4 Ω), or a deployed pretensioner remains unreplaced after a vehicle collision, causing an open circuit (>10 Ω).
  • 4Internal control unit fault: Damaged ACU internal CAN transceiver chip (e.g., NXP TJA1042) or MCU clock crystal drift causing communication timing errors. Voltage surges following battery depletion from long-term parking commonly cause this.
  • 5Software/configuration fault: ACU software version does not match the vehicle network configuration table (DID Config), or failure to clear crash data by executing the 'Crash Record Clear' procedure with the dedicated diagnostic tool locks the system in the fault state.
What To Do
  • 1
    Safety Preparation and Initial Inspection: Disconnect the 12V battery negative terminal and wait 3 minutes for the SRS capacitor to discharge. Use VDS or Launch X431 to read the complete DTC snapshot and check for accompanying B17A200 (crash lock) or B17A400 (hardwire fault) codes. Check the vehicle for previous collision repairs. Visually inspect the airbag module and pretensioner connectors for signs of deployment.
  • 2
    Power and ground verification: Reconnect the battery and turn the ignition ON. Measure the voltage at ACU connector pin 1 (constant B+) and pin 9 (IGN); voltage must be 11-14V. Measure the resistance between the ground pins (usually pins 10/11) and the vehicle body; resistance must be <1Ω. If the voltage is abnormal, check instrument panel power distribution box fuses F1/9 and F2/3, and ground point G301.
  • 3
    CAN bus physical layer check: Use an oscilloscope to measure the ACU CAN-H (pin 6) and CAN-L (pin 14) waveforms. The dominant level should be CAN-H 3.5V / CAN-L 1.5V, and the recessive level 2.5V. Disconnect power and measure the termination resistance. Total vehicle resistance should be approximately 60Ω (two 120Ω resistors in parallel). If resistance is abnormal, inspect the wiring harness section by section for short or open circuits.
  • 4
    Pretensioner circuit check: Disconnect the pretensioner connector (driver side usually below the B-pillar, passenger side behind the glovebox). Measure the pretensioner resistance. The resistance should be 2.0-2.4Ω. If open or shorted, replace the pretensioner assembly. Check the connector pins for oxidation (typically green copper corrosion). Clean with electronic contact cleaner if necessary.
  • 5
    Module replacement and calibration: If the wiring harness and pretensioner are normal, replace the ACU control unit (some models require VIN matching). After installing the new module, perform 'SRS system initialization' and 'crash sensor calibration'. Use the diagnostic tool to clear the DTC and perform a simulated crash test (use a dedicated resistor load instead of the airbag; never measure the airbag connector directly). Confirm B17A300 does not return and the instrument warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM SRS warning light on after driving through water, with DTC B17A300

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2019 BYD Qin PRO DM, 30,000 km. After driving through heavy flood water, the airbag warning light on the dash stayed on permanently. Scanned DTCs B17A300 and B17A200. Found the waterproof seal on the right front floor harness connector below the A-pillar was damaged. The CAN-H and CAN-L pins had oxidised, causing an intermittent short. Cleaned the oxide layer, applied conductive grease, and replaced the waterproof seal. CAN resistance now measured normal. Performed the collision record clear procedure. Fault resolved.
BYD DTC AI Analysis

Yuan PLUS communication fault caused by unreplaced pretensioner after accident repair

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2022 BYD Atto 3. After front collision repairs, the instrument cluster showed 'Safety System Fault'. DTCs: B17A300 (SRS CAN fault) and B165C00 (driver pretensioner open circuit). Inspection found the driver's seat belt pretensioner had deployed in the accident, but the repair shop only replaced the front bumper without replacing the seat belt assembly. The open pretensioner circuit caused the ACU to report a communication fault. Replaced the driver seat belt with force limiter assembly, performed online configuration and calibration. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Qin Pro (Fuel): Loose ACU power supply connection causing intermittent fault

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2018 BYD Qin Pro petrol. Airbag warning light illuminated intermittently while driving. Scan tool showed history DTC B17A300. Checked ACU power supply; found ignition voltage dropped below 8V on rough roads. Traced wiring to loose F2/3 fuse holder below the dashboard, causing intermittent ACU reset and CAN communication timeout. Replaced instrument panel distribution box assembly and tightened all connectors. Fault did not recur.
BYD DTC AI Analysis

Software mismatch caused SRS system lockout

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After replacing the ACU module at the dealership, the 2019 Qin Pro DM showed an airbag fault on the instrument cluster. Read DTCs B17A300 and U0155 (lost communication with ACU). The new module had the correct part number but software version V1.2; the vehicle requires V2.1 to support the new network architecture. Used the BYD dedicated diagnostic tool to perform online programming, updated the ACU software to the latest version, wrote the VIN, and matched the configuration word. System 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.