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 — Atto 3

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 Atto 3 indicates high resistance in the right knee airbag squib circuit, which can reduce or prevent proper airbag deployment. This is a medium-severity SRS fault and will usually turn on the airbag warning light. Common causes include a loose, corroded, or damaged connector under the passenger-side dashboard, or an internal knee airbag igniter fault. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Poor contact at the right knee airbag wiring harness connector: Long-term vibration causes backed-out pins, oxidation, corrosion, or looseness at the airbag connector under the passenger-side dashboard, increasing contact resistance (common in vehicles with water ingress or operated in high-humidity environments).
  • 2Knee airbag module internal igniter fault: Bridge wire oxidation, moisture ingress, or manufacturing defects inside the airbag igniter cause resistance to drift higher, indicating a fault in the airbag module itself.
  • 3Partial open circuit or hidden high resistance in the wiring harness: 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 by cross-checking).
  • 5Damage from improper previous repairs: Directly measuring the airbag igniter 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 igniter resistance.
What To Do
  • 1
    Connect BYD VDS2000 or a dedicated diagnostic tool. Read DTC B1728-00 and freeze frame data. Record ambient temperature, voltage, and vehicle status at the time of the fault. Confirm whether the fault is intermittent.
  • 2
    Perform an airbag system self-diagnosis and check for accompanying fault codes (if B1728-00 appears with communication fault codes, troubleshoot 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. Check the right knee airbag wiring harness connector (usually yellow) for looseness, water ingress, or pin oxidation.
  • 4
    Connect the dedicated airbag load substitute (2Ω_dummy_load) to the wiring harness side, power on the system, and read the fault code: if the fault code changes to "normal resistance/short to ground", wiring harness continuity is normal and the fault is in the airbag module; if the code still indicates high resistance, the fault is in the wiring harness or the ACU.
  • 5
    If you suspect a wiring harness fault, use a multimeter to measure the continuity resistance between the harness terminal and the corresponding ACU pin (should be <1Ω) and the insulation to ground/power. Inspect the harness fixing points at the dashboard crossmember for wear.
  • 6
    If the airbag module is confirmed faulty, replace the right knee airbag module (Note: Do not drop or strike the new module during transport and installation. Do not 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 confirm the SRS warning light turns off normally and B1728-00 does not reappear.
  • 8
    Perform a final road test, simulate vehicle vibration conditions, verify the fault does not return, 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 Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.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.
Common problems reported for Atto 3 →Atto 3 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.