AU
B1727

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

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.

The B1727 fault code on the Seal U indicates a medium-severity SRS fault where the passenger knee airbag squib circuit resistance is too low. This may be caused by an internal short in the knee airbag igniter or damaged wiring shorting to ground. The airbag warning light may come on and airbag protection could be reduced. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Internal inter-turn short circuit in the right knee airbag igniter: Damaged insulation on the airbag's 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 harness from the SRS ECU to the right knee airbag (typically a yellow sleeve) contacts vehicle body metal, causing current shunting and a low resistance reading.
  • 3Connector terminal short circuit or bridging: Bent pins, metallic foreign objects bridging the terminals, or water ingress corrosion in the front passenger knee airbag connector (usually under the dashboard) reduce resistance between the terminals.
  • 4SRS ECU internal sampling circuit fault: A faulty internal ECU ADC (analog-to-digital converter) or voltage divider resistor falsely reports low resistance. The actual airbag resistance is normal, but the ECU detects a low value.
  • 5Modification or repair damage: Tools pierce or crush the airbag wiring harness during dash cam installation, audio system modification, or floor trim removal and installation, causing internal conductors to contact.
What To Do
  • 1
    Safety precautions and initial checks: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to allow the SRS capacitor to discharge). Use a megohmmeter to verify no residual high voltage remains in the system. Read the SRS data stream and record the specific current resistance value of the right knee airbag (e.g., 0.3Ω, 0.8Ω, etc.) to determine whether 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 matter. Disconnect the airbag connector. Use a special probe to measure the resistance between the airbag-side terminals (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 lies in the wiring harness or ECU. If the resistance remains low, replace the airbag module.
  • 3
    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 harness (should be >10MΩ) and measure continuity between both ends of the harness (should be <1Ω). If testing reveals a short to ground, 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 wiring harness and airbag resistance are normal, test with a known-good SRS ECU (Note: Match the part number and software version to prevent VIN locking issues). If the fault clears, replace the SRS ECU, rewrite the vehicle model configuration, and perform coding.
  • 5
    System reset and verification: After completing repairs, clear the fault codes, reconnect the battery, and turn the ignition switch to the ON position. Confirm 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 (use a dedicated resistor instead of the airbag, or execute 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 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.