AU
B1727

DTC B1727 indicates the SRS (Supplemental Restraint System) ECU detects Passenger Knee Airbag squib circuit resistance below the calibrated threshold (typically <1 — Atto 3

Safety System

DTC B1727 indicates the SRS (Supplemental Restraint System) ECU detects Passenger Knee Airbag squib circuit resistance below the calibrated threshold (typically <1.0Ω or <1.5Ω, depending on vehicle calibration).

This fault represents an abnormally low impedance in the squib circuit.

Potential causes include an internal short circuit in the squib coil, a wiring harness short to ground, a short between connector terminals, or a faulty internal sampling circuit within the SRS ECU.

Low resistance typically indicates an abnormal current path, causing the SRS system to identify a risk of unintended deployment or monitoring failure.

The system subsequently illuminates the airbag warning light and may disable the entire airbag system.

This results in the loss of knee protection during a collision and disrupts the normal deployment logic of other airbags.

4
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Right knee airbag igniter internal inter-turn short circuit: Damaged insulation on the internal igniter coil reduces the effective number of turns and lowers resistance. Airbag aging, moisture ingress, or manufacturing defects usually cause this fault.
  • 2Harness short to ground: Damaged insulation on the wiring harness (usually in a yellow sleeve) from the SRS ECU to the right knee airbag contacts vehicle body metal, shunting current and causing a low resistance reading.
  • 3Connector terminal short circuit or bridging: Bent pins, metallic foreign objects bridging the terminals, or water ingress corrosion in the airbag connector in the front passenger-side knee area (usually under the dashboard) reduce impedance between terminals.
  • 4SRS ECU internal sampling circuit fault: A faulty internal ECU ADC (analog-to-digital converter) or voltage divider resistor causes a false low resistance report. The actual airbag resistance is normal, but the ECU detects a low value.
  • 5Modification or repair damage: Tools piercing or crushing the airbag wiring harness during dash cam installation, audio system modification, or floor trim removal/installation, causing contact between internal conductors.
What To Do
  • 1
    Safety precautions and initial checks: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to discharge the SRS capacitor). Use a megohmmeter to confirm the system has no residual high voltage. Read the SRS data stream and record the specific current resistance value of the right knee airbag (e.g., 0.3Ω, 0.8Ω) to determine if the fault is intermittent or continuous.
  • 2
    Visual inspection and connector check: Inspect the yellow SRS connector in the front passenger knee airbag area (lower right side of the dashboard) for looseness, water ingress, or foreign matter. Disconnect the airbag connector. Measure the resistance between the airbag-side terminals using a special probe. Do not insert the probe directly into the terminals to prevent damage. Standard value: 1.5-3.5Ω. If the resistance is normal, the fault is in the wiring harness or ECU. If the resistance is low, replace the airbag module.
  • 3
    Wiring harness insulation test: Disconnect the SRS ECU connector (usually under the centre console or near the gear selector). Measure the resistance to earth at both ends of the right knee airbag wiring harness (should be >10 MΩ), and measure the continuity between both ends of the wiring harness (should be <1 Ω). If a short to earth exists, strip the wiring harness in sections to locate the damaged insulation. After repair, apply double-layer heat-shrink tubing to protect the insulation.
  • 4
    ECU replacement verification: If the wiring harness and airbag resistance are normal, perform a substitution test using a known-good SRS ECU (Note: ensure the part number and software version match to prevent VIN lock issues). If the fault clears after replacement, replace the SRS ECU, then rewrite the vehicle configuration and perform coding.
  • 5
    System reset and verification: After repair, clear the fault codes, reconnect the battery, and turn the ignition switch to ON. Verify the airbag warning light turns off after the self-check. Use the diagnostic tool to perform 'SRS System Configuration Confirmation' and 'Crash Sensor Calibration' (if applicable). Perform a simulated crash test (using a dedicated resistor instead of the airbag, or executing the 'Component Test' function via the diagnostic tool) to verify system recovery.
Real-World Cases
BYD DTC AI Analysis

BYD Tang DM-i knee airbag internal short caused B1727

✓
2021 BYD Tang DM-i with 32,000 km. The instrument panel displayed 'Check SRS System'. The scan tool read DTC B1727 (right knee airbag resistance too low). Live data showed resistance at 0.4Ω (normal 2.0Ω). Disconnected the negative battery terminal, removed the trim panel under the passenger glovebox, and disconnected the knee airbag connector. Direct measurement across the airbag terminals still showed 0.4Ω, indicating an internal short in the airbag igniter. Replaced the right knee airbag assembly (part number: KFB-5820400). After clearing the DTC, resistance returned to 2.1Ω, resolving the fault. Root cause: moisture ingress into the airbag propellant caused coil insulation breakdown.
BYD DTC AI Analysis

BYD Song MAX wiring harness chafed, short to earth

✓
2019 BYD Song MAX. After accident repairs, DTC B1727 set. A non-authorised repair shop had replaced the windscreen and dashboard. Inspection found the instrument panel crossmember fixing clip had pinched the RH knee airbag wiring harness, exposing copper that shorted to body ground. Airbag resistance measured 2.3Ω (normal), but harness-to-ground resistance measured 0.2Ω. Rerouted the harness away from sharp metal edges, repaired the damaged section with double-layer heat shrink tubing, and added a rubber protective sleeve at the clip. Cleared the DTC; it has not returned.
BYD DTC AI Analysis

Water ingress and corrosion in the BYD Yuan EV connector

✓
2019 BYD Yuan EV535. Airbag warning light came on after driving through water. Retrieved DTC B1727, accompanied by B1726 (left knee airbag resistance abnormal). Inspection found water staining on the passenger floor. The knee airbag connector under the passenger-side carpet had internal green copper corrosion with electrolytic conduction between terminals. Cleaned the connector terminals, flushed with electronic cleaner, dried, and applied conductive protectant. Also traced the leak to a disconnected AC drain hose, repaired it, and cleared the fault. For vehicles driven through water, recommend thorough inspection of SRS connector sealing.
BYD DTC AI Analysis

BYD Qin Pro SRS ECU falsely reporting low resistance

✓
2020 Qin Pro petrol, no accident history. DTC B1727 appeared suddenly. Measured right knee airbag resistance at 1.8Ω (normal). Wiring harness insulation was good, but the fault code appeared intermittently. Compared data stream between left and right knee airbags and found the right side reading fluctuating between 0.8–1.2Ω, while the left remained stable at 1.9Ω. Suspected internal sampling circuit fault in the SRS ECU. Replaced the SRS control module (requiring vehicle parameter reconfiguration and VIN programming), which eliminated the fault completely. Disassembly of the original ECU revealed a cold solder joint on the internal voltage-dropping resistor, causing unstable sampling voltage.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.