AU
B1728

DTC B1728 indicates the SRS (Supplemental Restraint System) ECU detects the right knee airbag squib circuit resistance exceeds the calibrated threshold (the normal range is typically 1 — Qin Plus

Safety System

DTC B1728 indicates the SRS (Supplemental Restraint System) ECU detects the right knee airbag squib circuit resistance exceeds the calibrated threshold (the normal range is typically 1.5-3.0Ω; actual thresholds vary by vehicle software version, generally triggering this code at >3.5Ω).

Electrically, excessive resistance indicates a high-impedance point in the circuit, resulting from increased contact resistance, a partial open circuit, or squib aging.

This hard or intermittent fault causes the SRS to enter a degraded mode.

During a collision, the airbag may fail to inflate, experience delayed inflation, or deploy with insufficient energy, failing to effectively protect the occupant's knees and lower limbs.

Simultaneously, the instrument cluster airbag warning light remains illuminated, and the system may lock other airbag circuits.

4
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Oxidized pins, terminal back-out, or insufficient contact pressure at the right knee airbag module connector (usually located below the steering wheel or inside the right side of the dashboard), resulting in increased contact resistance.
  • 2Corroded or deformed shorting bar in the clock spring or knee airbag wiring harness fails to fully disengage or makes poor contact, introducing additional resistance.
  • 3Microscopic fractures or carbonization of the internal firing wire in the right knee airbag igniter (squib), causing the resistance value to drift upward (common in older vehicles or after minor collisions without airbag deployment)
  • 4Internal open circuit, pinching, or partially broken copper strands in the wiring harness from the SRS ECU to the knee airbag, reducing the effective conductive cross-sectional area.
  • 5SRS ECU internal sampling circuit fault or reference resistor drift causing a false code (less common; confirm after ruling out external wiring issues)
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes (5 minutes for some models) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Diagnostic confirmation: Use VDS or a dedicated diagnostic tool to read the fault code. Check the resistance value in the freeze frame data (Freeze Data) to confirm the fault is a current fault (Current), not a history fault (History).
  • 3
    Visual inspection: Remove the right knee airbag (usually requires removing the lower dashboard trim panel). Inspect the connector for water ingress, the pins for oxidation or blackening, and the wiring harness for signs of crushing or chafing.
  • 4
    Resistance measurement: Use a calibrated digital multimeter to measure resistance directly across the airbag inflator terminals. Normal resistance is 1.5-3.0 Ω. If resistance is >3.5 Ω, the airbag module is faulty.
  • 5
    Circuit continuity test: Disconnect the SRS ECU connector. Measure the wiring harness resistance from the ECU side to the airbag side (must be <1Ω) and check the insulation to ground (must be >1MΩ). Inspect the clock spring closely (if the knee airbag circuit routes through it).
  • 6
    Shorting bar check: Inspect the shorting bar (copper strip) inside the airbag connector for deformation or corrosion. Ensure the shorting bar opens fully after connection and does not contact adjacent pins.
  • 7
    Repair/Replace: If the wiring harness is faulty, repair the harness (soldering + heat-shrink tubing). If the airbag unit resistance exceeds the limit, replace the right knee airbag module (Note: store the new module stationary to prevent electrostatic discharge).
  • 8
    System reset: After reassembly, connect the battery, clear the fault codes, and perform an SRS system self-check (normally, the warning light illuminates for 6 seconds after turning the ignition switch ON and then turns off, indicating normal operation). Perform a road test to verify.
Real-World Cases
BYD DTC AI Analysis

Oxidation on the right knee airbag connector of the BYD Tang DM caused high resistance.

✓
A 2021 Tang DM with 32,000 km displayed 'Check SRS System' on the instrument cluster. Retrieved DTC B1728; live data showed 4.2 Ω. Removed the right knee airbag connector near the floor and found water ingress had caused green corrosion on the pins. Cleaned the pins with electronic cleaner, applied conductive grease, and reassembled. Resistance returned to 2.1 Ω and the fault cleared. Recommend checking the vehicle seals and drain holes.
BYD DTC AI Analysis

Intermittent B1728 due to internal wire breakage in Yuan EV clock spring

✓
2019 Yuan EV with intermittent airbag warning light. DTC B1728 intermittent. Multiple static resistance measurements were normal, but when turning the steering wheel, resistance fluctuated above 5 Ω. Disassembled the clock spring (spiral cable) and found micro-cracks in the internal flexible PCB and fractured copper foil traces. Replaced the clock spring assembly; fault disappeared. Tip: Knee airbag wiring sometimes routes through the clock spring; check downstream circuits.
BYD DTC AI Analysis

Song MAX knee airbag module internal igniter aging

✓
2019 Song MAX: Replaced the knee airbag with an aftermarket unit during accident repairs. After six months, the system logged DTC B1728. Measured the aftermarket airbag resistance at 3.8Ω, exceeding the specification. Installed a genuine airbag module (resistance 2.0Ω) and the fault cleared. The aftermarket igniter filament material or diameter did not meet standards, causing resistance drift after extended use. Always use OEM parts for airbag components.
BYD DTC AI Analysis

The dashboard bracket crushed the Qin Pro wiring harness, causing high resistance.

✓
A 2020 BYD Qin Pro developed DTC B1728 after an aftermarket audio installation. Inspection found that during dashboard removal for the modification, the edge of the instrument panel metal bracket had pinched the right knee airbag wiring harness. Internal copper wires partially fractured but did not break completely, creating a high-resistance condition (measured at 8.5Ω). Stripping back the harness revealed three broken copper wires. After resoldering and reinforcing the insulation, resistance returned to 1.8Ω. Work involving dashboard removal requires special attention to wiring harness routing.
Other Qin Plus 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.