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 8

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 8 indicates high resistance in the right knee airbag squib circuit, usually under the passenger-side dashboard. This medium-severity SRS fault turns on the airbag warning light and may reduce crash protection if the airbag cannot deploy correctly. Common causes include a loose, corroded, or oxidised connector, or a faulty knee airbag igniter. 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 — Atto 8
  • 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 driven through water or operated in high-humidity environments).
  • 2Knee airbag module internal igniter fault: Oxidation, moisture ingress, or manufacturing defects in the airbag igniter internal bridge wire cause the resistance value to drift higher, indicating a faulty airbag module.
  • 3Partial open circuit or hidden high resistance in 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 instead of a complete open circuit.
  • 4SRS control unit (ACU) sampling circuit fault: A fault in the ACU internal sampling resistor, A/D conversion circuit, or connector 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 igniter with a standard multimeter resistance setting during non-standard servicing (without using a dedicated load, or applying excessive deployment current) damages the internal bridge wire and increases igniter resistance.
What To Do
  • 1
    Connect the BYD VDS2000 or a 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 whether the fault is intermittent.
  • 2
    Run the airbag system self-diagnosis and check for accompanying fault codes (if B1728-00 occurs with communication fault codes, 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. Check the right knee airbag wiring harness connector (usually yellow) for looseness, water ingress, or oxidized pins.
  • 4
    Connect the special airbag load simulator (2Ω_dummy_load) to the wiring harness side, power on again, and read the fault code. If the fault code changes to "normal resistance/short to ground", wiring harness continuity is normal and the airbag module is faulty. If the fault code still indicates high resistance, the wiring harness or ACU is faulty.
  • 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). Check the insulation to ground and power supply. Inspect the harness fixing points at the instrument panel crossmember for wear.
  • 6
    If testing confirms a faulty airbag module, replace the right knee airbag module (Note: Do not drop or impact 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 verify the SRS warning light turns off normally and B1728-00 does not return.
  • 8
    Perform a final road test to simulate vehicle vibration conditions. Verify the fault does not recur. 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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 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.