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 — Qin Plus

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.

4
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Internal short circuit in the knee airbag igniter: Manufacturing defects or moisture ingress damage the insulation on the igniter bridge wire inside the airbag assembly, creating an internal short circuit.
  • 2Physical damage to the wiring harness: During dashboard removal and installation, carpet replacement, or front passenger glovebox repair, the metal frame crushes or cuts the knee airbag wiring harness (usually in a yellow conduit), 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 center console or front passenger seat) during vehicle wading, A/C condensate leakage, or floor cleaning, causing an electrolytic short circuit between 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 system 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 freeze frame data shows abnormal resistance (typically <0.5Ω). Disconnect the 12V 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). Check the yellow connector of the right knee airbag module (located on the frame under the glove box) for looseness, water ingress, or obvious signs of corrosion. Inspect the wiring harness for crush damage from the frame edge or punctures from screws.
  • 3
    Segmented resistance measurement: Use a dedicated airbag tester (low-current mode <10mA) or a digital multimeter (resistance setting only). Disconnect the airbag connector and measure the following separately: ① Resistance between airbag-side terminals (normal: 2.0-3.0Ω); ② Harness-side resistance to ground (should be infinite); ③ Resistance between harness-side terminals (should be infinite, ruling out harness short circuits).
  • 4
    Wiring harness circuit check: If the airbag resistance is normal, use a jumper wire to bridge the harness-side connector, then measure harness continuity and resistance at the SRS module connector. Inspect the protective rubber sleeves where the harness passes through the firewall and floor channel for damage. Carefully check the harness above the front passenger footwell for interference with the A/C duct metal bracket.
  • 5
    Insulation repair or replacement: If the wiring harness is damaged, repair it using heat-shrink tubing or insulating tape and resecure the harness. Maintain a >20mm clearance from sharp metal edges. If the airbag module resistance is abnormal (<1.0Ω), you must 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 the SRS system self-test (usually requires turning the ignition switch to the ON position and waiting 6 seconds) and confirm 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" (if 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 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.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.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.
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.