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 8

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 8
  • 1Internal inter-turn short circuit in the right knee airbag igniter: Damaged insulation on the internal airbag ignition 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: The harness from the SRS ECU to the right knee airbag (usually with yellow sleeving) has damaged insulation and contacts vehicle body metal, causing current shunting and abnormally low resistance.
  • 3Connector terminal short circuit or bridging: Bent pins, metal foreign objects bridging the terminals, or water ingress corrosion in the front passenger knee area airbag connector (usually under the dashboard) reduce impedance between the terminals.
  • 4SRS ECU internal sampling circuit fault: A fault in the ECU internal ADC (analog-to-digital converter) or voltage divider resistor causes a false low resistance reading; 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 dashcam installation, audio system modification, or floor trim removal and installation, causing internal conductors to make contact.
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 verify 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 dashboard) for looseness, water ingress, or foreign objects. Disconnect the airbag connector. Use a dedicated probe to measure the resistance between the airbag-side terminals (do not insert probes directly into the terminals to prevent damage). Standard value: 1.5-3.5Ω. If the resistance is normal, the fault lies in the wiring harness or ECU. If the resistance remains low, replace the airbag module.
  • 3
    Wiring harness insulation test: Disconnect the SRS ECU connector (usually located under the centre console or near the gear selector). Measure the resistance to ground at both ends of the right knee airbag wiring harness (should be >10 MΩ) and measure continuity between both ends of the harness (should be <1 Ω). If a short to ground exists, strip back the harness in sections to locate the damaged insulation. Repair the damage and apply double-layer heat-shrink tubing for insulation protection.
  • 4
    ECU replacement verification: If both the wiring harness and airbag resistance are normal, perform a substitution test using a known-good SRS ECU (Note: Verify the part number and software version match to prevent VIN locking issues). If the fault clears after replacement, replace the SRS ECU, then write the vehicle configuration and perform coding.
  • 5
    System reset and verification: After repair, clear the fault codes, reconnect the battery, turn the ignition switch to ON, and confirm the airbag warning lamp turns off after the self-check. Use a diagnostic tool to execute '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 normal system operation.
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 8 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.