AU
B1728-00

DTC B1728-00 indicates the Airbag Control Unit (ACU) detects the squib circuit resistance of the right knee airbag (usually located beneath the passenger-side dashboard) exceeds the calibrated threshold — Qin Plus

Safety System

DTC B1728-00 indicates the Airbag Control Unit (ACU) detects the squib circuit resistance of the right knee airbag (usually located beneath the passenger-side dashboard) exceeds the calibrated threshold.

The normal range is typically 1.5-3.0 Ω, varying by vehicle calibration.

High resistance generally means >3.5 Ω or a near open-circuit condition.

This is an actively monitored SRS fault.

The ACU continuously monitors the airbag squib circuit current and resistance to determine circuit integrity.

High resistance indicates a high-resistance connection in the squib circuit.

This may result in insufficient ignition energy during a collision, preventing proper airbag deployment.

In extreme cases, the system detects an open-circuit fault and triggers a secondary lockout, compromising the entire airbag system operation.

This fault illuminates the instrument cluster SRS warning lamp and forces the vehicle passive safety system into a degraded protection mode.

The B1728-00 fault code on the Qin Plus indicates high resistance in the right knee airbag squib circuit, which can reduce airbag deployment reliability and puts the SRS in a degraded protection mode. Severity is medium. Common causes include a loose, corroded, or oxidized passenger-side airbag connector, or an internal igniter fault in the knee airbag module. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Qin Plus
  • 1Poor contact at the right knee airbag wiring harness connector: Long-term vibration causes terminal back-out, oxidation, corrosion, or looseness at the airbag connector under the passenger-side dashboard, increasing contact resistance (common in vehicles driven through water or operated in high-humidity environments).
  • 2Knee airbag module internal igniter fault: Oxidation of the igniter bridge wire, moisture ingress, or manufacturing defects cause the resistance value to increase. This indicates a faulty airbag module.
  • 3Wiring harness partial open circuit or hidden high resistance: The harness connecting the ACU to the right knee airbag has partially broken internal copper wires or loose crimp points where it passes through the instrument panel frame, floor, or hinge, creating a high-resistance path rather than a complete open circuit.
  • 4SRS control unit (ACU) sampling circuit fault: Faulty sampling resistor, A/D conversion circuit, or connector inside the ACU related to the right knee airbag channel causes a false high resistance reading (rule out via cross-checking).
  • 5Damage from improper previous repairs: Directly measuring the airbag inflator with a standard multimeter resistance setting during non-standard operations (failing to use a dedicated load or applying excessive deployment current) damages the internal bridge wire and increases resistance.
What To Do
  • 1
    Connect the BYD VDS2000 or dedicated diagnostic tool. Read DTC B1728-00 and freeze frame data. Record the ambient temperature, voltage, and vehicle status at the time of the fault. Confirm if the fault is intermittent.
  • 2
    Perform the airbag system self-diagnosis and check for accompanying fault codes (if B1728-00 appears with communication faults, inspect the CAN wiring harness first).
  • 3
    Disconnect the battery negative terminal and wait at least 3 minutes (to fully discharge the SRS capacitor). Remove the front passenger lower dashboard trim panel and inspect the right knee airbag wiring harness connector (usually a yellow connector) for looseness, water ingress, or oxidized pins.
  • 4
    Connect the dedicated airbag load substitute (2Ω_dummy_load) to the wiring harness side, restore power, and read the fault code: if the fault code changes to 'Normal resistance/short to ground', the wiring harness continuity is normal and the fault is in the airbag module; if it still indicates high resistance, the fault is in the wiring harness or the ACU.
  • 5
    If suspecting a wiring harness fault, use a multimeter to measure continuity resistance between the harness terminal and the corresponding ACU pin (should be <1Ω) and check insulation to ground/power. Carefully inspect the harness mounting points at the instrument panel crossmember for wear.
  • 6
    If confirming an airbag module fault, replace the right knee airbag module. (Note: Do not drop or strike the new module during transport and installation, and never measure its resistance.)
  • 7
    After repairing or replacing the faulty component, reconnect all connectors and the battery. Clear the fault code, perform an ignition cycle (IGN ON-OFF three times), and verify the SRS warning light turns off normally and B1728-00 does not return.
  • 8
    Perform a final road test, simulate vehicle vibration conditions, confirm the fault does not recur, and complete the airbag repair record form.
Real-World Cases
BYD DTC AI Analysis

Water ingress caused oxidation of the right knee airbag connector on the BYD Qin EV300, resulting in high resistance.

✓
SRS warning light came on after the vehicle waded through water. Read DTC B1728-00. Inspection revealed signs of water intrusion in the front passenger footwell. The right knee airbag connector (under the HVAC duct) had a degraded seal and green copper corrosion on its pins. Contact resistance measured 5.8Ω. Cleaned the pins with electronic cleaner, applied conductive grease, and replaced the connector seal. Fault resolved. Recommend checking connector sealing condition on vehicles from the same production batch.
BYD DTC AI Analysis

Qin 80 airbag module internal resistance drift replacement case

✓
Customer reported the SRS light illuminating intermittently. B1728-00 was a historic fault code. Checked wiring harness connectors – no abnormalities. Verified harness continuity with a load simulator – normal. Measured old airbag module resistance: 4.2Ω (exceeds 3.0Ω upper limit). Diagnosed internal igniter degradation in the airbag module. Replaced right knee airbag module; resistance restored to 2.1Ω, fault resolved. Root cause: prolonged high-temperature operation caused oxidation of the internal bridge wire.
BYD DTC AI Analysis

Dashboard Wiring Harness Intermittent Partial Open Circuit Misdiagnosis Case

✓
The fault recurred intermittently after multiple repairs. The airbag module had been mistakenly replaced with no effect. Finally traced the fault to the instrument panel crossbeam mounting clip pinching the wiring harness, which had fractured 90% of the internal copper strands, leaving only minimal continuity. Resistance was normal when cold but rose to 3.8Ω when hot or under vibration, triggering the fault code. Repaired the harness and rerouted it to avoid the interference; fault resolved. Tip: These faults require a 'wiggle test' (measuring while moving the harness) to detect.
BYD DTC AI Analysis

ACU Sampling Circuit False Positive Diagnosis

✓
The vehicle has no accident history. Fault codes B1728-00 and B1725-00 (left knee airbag) appeared simultaneously. Both left and right knee airbag harnesses and modules tested normal, pointing to an ACU fault. Connecting the right knee airbag harness to a spare ACU channel (e.g., a reserved seat channel) transferred the fault code to that channel, confirming a damaged sampling resistor in the ACU's right knee airbag detection channel. Replacing the airbag control unit resolved the issue.
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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.