AU
B1727-00

This DTC indicates the Airbag Control Unit (ACU) detects the right knee airbag (front passenger knee airbag) squib circuit resistance is below the system calibration threshold (typically <1 — Seal U

Safety System

This DTC indicates the Airbag Control Unit (ACU) detects the right knee airbag (front passenger knee airbag) squib circuit resistance is below the system calibration threshold (typically <1.0Ω; normal range: 2.0-3.0Ω).

Low resistance indicates an abnormally low-resistance path in the circuit.

A short to ground, a wire-to-wire short, or an internal short in the airbag squib typically causes this condition.

This fault presents two severe risks: first, short-circuit current can trigger unintended airbag deployment, injuring occupants; second, excessive circuit current during a collision can burn the contacts, preventing airbag deployment.

This fault constitutes an active safety system failure.

The vehicle remains drivable, but the airbag system enters degraded protection mode.

During a collision, the right knee airbag and any other airbags with related faults will fail to deploy.

The B1727-00 fault code on the Seal U indicates low resistance in the front passenger right knee airbag squib circuit. This is a medium-severity safety fault because it can cause unintended deployment or prevent the airbag from deploying in a crash. Common causes include an internal short in the knee airbag igniter or damaged wiring near the dashboard/glove box. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Knee airbag igniter internal short circuit: Manufacturing defects or moisture ingress damage the insulation of the igniter bridge wire inside the airbag assembly, causing an internal short circuit.
  • 2Wiring harness physical damage: The metal frame crushed or cut the knee airbag wiring harness (usually in a yellow sleeve) during dashboard removal and installation, carpet replacement, or front passenger glove box repair, causing a short to ground.
  • 3Connector water ingress and corrosion: Water enters the airbag connector under the front passenger floor (usually located under the lower centre console or beneath the front passenger seat) during vehicle wading, A/C condensate leaks, or floor cleaning, causing an electrolytic short circuit between the terminals.
  • 4SRS control module internal drive circuit fault: ACU internal ignition drive transistor breakdown or sampling resistor circuit fault, causing a false low resistance reading.
  • 5Improper operation or modification: Unqualified personnel accidentally pierce the airbag wiring harness while installing a dash cam, ambient lighting, or audio equipment in the front passenger area, causing the copper core to short to ground.
What To Do
  • 1
    Safety preparation and diagnostic confirmation: Use the BYD ED400/ED600 diagnostic tool to read DTCs. Confirm B1727-00 is a current fault and the freeze frame data shows abnormal resistance (usually <0.5Ω). Disconnect the battery negative terminal and wait at least 90 seconds for the SRS capacitor to fully discharge.
  • 2
    Visual inspection and location: Remove the passenger-side lower dashboard trim panel (knee bolster). Inspect the right knee airbag module yellow connector (located on the frame beneath the glovebox) for looseness, water ingress, or obvious corrosion. Inspect the wiring harness for crush damage from the frame edge or puncture damage from screws.
  • 3
    Segmented resistance measurement: Use a dedicated airbag tester (low-current mode <10mA) or a digital multimeter (resistance mode only). Disconnect the airbag connector and measure the following separately: ① resistance between the airbag-side terminals (normal: 2.0-3.0Ω); ② resistance from the harness side to ground (should be infinite); ③ resistance between the harness-side terminals (should be infinite, to rule out a short circuit between harness wires).
  • 4
    Harness circuit inspection: If the airbag resistance is normal, bridge the harness-side connector with a jumper wire and measure harness continuity and resistance at the SRS module connector. Inspect the protective rubber sleeves for damage where the harness passes through the firewall and floor channel. Carefully inspect the harness above the front passenger footwell for interference with the air-conditioning duct metal bracket.
  • 5
    Insulation repair or replacement: If the wiring harness is damaged, repair it with heat-shrink tubing or insulating tape. Re-secure the harness to maintain a >20mm clearance from sharp metal edges. If the airbag assembly resistance is abnormal (<1.0Ω), replace the right knee airbag assembly (part number usually starts with 57). Never repair the airbag.
  • 6
    System reset and verification: Restore all connections, reconnect the battery, and clear the fault code. Perform an SRS system self-test (usually requires turning the ignition switch to the ON position and waiting 6 seconds) and verify B1727-00 does not return. Perform a wiggle test: Gently wiggle the knee airbag wiring harness by hand and verify the 'right knee airbag resistance' in the diagnostic tool data stream remains stable within the 2.0-3.0Ω range.
  • 7
    Final confirmation: Use the diagnostic tool to perform 'Crash Sensor Calibration' or 'SRS System Configuration' (after replacing the airbag or ACU). Confirm the instrument cluster SRS warning light turns off and the system status displays 'Normal'.
Real-World Cases
BYD DTC AI Analysis

Qin EV450 right knee airbag short circuit after wading

✓
A 2018 BYD Qin EV450 displayed "Check SRS System" on the dashboard after driving through floodwater during heavy rain. The scan tool retrieved B1727-00 (current fault). Inspection found water pooling in the front passenger footwell. After removing the front passenger seat and lower centre console trim, the knee airbag connector (located inside the front passenger seat rail) showed visible water staining and the terminals had green corrosion. Cleaning and drying the connector still yielded low resistance readings; further inspection revealed water had penetrated the airbag igniter. Replaced the right knee airbag assembly and resealed the floor waterproofing to resolve the fault.
BYD DTC AI Analysis

Qin 80 wiring harness crushed after accident repair

✓
One month after a front collision repair, this 2017 Qin 80 set DTC B1727-00. During the repair, the technician replaced the dashboard and passenger airbag but did not properly secure the knee airbag harness. Inspection found a screw at the dashboard frame mounting point had crushed the harness, damaging the insulation and causing a short to ground. They rerouted the harness, fastened it with dedicated clips, and made sure it had enough clearance from the frame edges. After repairing the insulation, the fault cleared; the airbag unit itself was not replaced.
BYD DTC AI Analysis

Qin 100 air conditioning water leak caused connector corrosion

✓
The owner reported that the airbag warning light occasionally illuminated. The scan tool retrieved historical fault code B1727-00. Inspection revealed a blocked passenger-side air conditioning evaporator drain hose, causing condensate to overflow onto the floor. The knee airbag connector, located above the passenger footwell, had been exposed to prolonged moisture, resulting in a micro-short between terminals. Cleaned the connector terminals, applied conductive anti-corrosion compound, unclogged the drain hose, and wrapped the connector with waterproof tape. Measured resistance returned to normal. One-month follow-up showed no recurrence.
BYD DTC AI Analysis

SRS module false alarm causing abnormal resistance

✓
2017 Qin EV300 frequently setting DTC B1727-00, but measured resistance of knee airbag and harness was normal (2.3Ω). Harness: no short circuits; airbag: intact. Suspected internal sampling circuit fault in SRS control module (ACU). Swapped in a known-good ACU from another vehicle for testing; fault transferred with the module. After reinstalling original ACU, fault returned. Replaced airbag control module and reconfigured vehicle VIN and crash sensor parameters. Fault permanently resolved.
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.