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 — Seal U

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 Seal U indicates high resistance in the right knee airbag squib circuit, usually under the passenger-side dashboard. This medium-severity SRS fault can reduce airbag protection and turn on the airbag warning light. Common causes include a loose, corroded, or oxidised connector, or an internal knee airbag igniter fault. 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 — Seal U
  • 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 used in high-humidity environments).
  • 2Knee airbag module internal igniter fault: Oxidation of the airbag igniter internal bridge wire, moisture ingress, or a manufacturing defect causes the resistance value to increase. This is a fault of the airbag module itself.
  • 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 rather than a complete open circuit.
  • 4SRS control unit (ACU) sampling circuit fault: A fault in the 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 igniter using a standard multimeter resistance setting during non-standard servicing (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 when the fault occurred. Determine if 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 12V battery negative terminal and wait at least 3 minutes (to ensure the SRS capacitor fully discharges). Remove the front passenger lower dashboard trim panel. Inspect the right knee airbag wiring harness connector (usually yellow) for looseness, water ingress, or oxidized pins.
  • 4
    Connect the special airbag load substitute (2Ω_dummy_load) to the wiring harness side, power on the vehicle, and read the fault codes. 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 the fault code 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 the continuity resistance between the harness terminal and the corresponding ACU pin (should be <1Ω) and check the insulation to ground and power. Carefully inspect the harness fixing points at the dashboard crossmember for wear.
  • 6
    If confirming an airbag module fault, replace the right knee airbag module (Note: Do not drop or impact the new module during transport and installation, and do not measure its resistance).
  • 7
    After repairing or replacing the faulty component, reconnect all connectors and the battery. Clear the fault code, perform ignition cycles (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 to simulate vehicle vibration conditions, verify 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 Seal U Fault Codes
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.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.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.