AU
B172B1B

DTC B172B1B indicates the Supplemental Restraint System (SRS) detected a communication loss between the left middle-row (second-row left-side) side airbag module and the SRS control unit (ACU) — Atto 3

Safety System

DTC B172B1B indicates the Supplemental Restraint System (SRS) detected a communication loss between the left middle-row (second-row left-side) side airbag module and the SRS control unit (ACU).

This fault indicates an open airbag module circuit with resistance exceeding the threshold (typically >10Ω), preventing the control unit from detecting the module.

Possible causes include a disconnected physical connector, broken wiring harness, faulty internal spiral cable (clock spring), or an open circuit within the module itself.

Because the SRS uses a dual-stage deployment circuit, this fault prevents the left middle-row side airbag from deploying during a collision.

It may also trigger a system self-check lockout, affecting the entire side airbag system.

The B172B1B fault code on the Atto 3 indicates a medium-severity SRS fault where communication is lost with the left second-row side airbag module, so that airbag may not deploy in a crash. Common causes include a loose or disconnected airbag connector under the seat or in the B-pillar, or a broken wiring harness from repeated bending. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1The airbag wiring harness connector under the seat or inside the B-pillar trim panel is loose, disconnected, or has poor contact, commonly caused by frequent seat adjustment or failing to fully engage the connector after B-pillar trim removal and installation.
  • 2Wiring harness fatigue fracture, especially harnesses routed through the seat frame or near the door hinge, where long-term bending and compression break the internal copper strands while the outer insulation remains intact.
  • 3Internal airbag module open circuit; abnormal internal squib resistance (open circuit or excessive resistance). Usually results from module aging, previous moisture exposure, or electrostatic discharge.
  • 4Corrosion or poor contact at the SRS control unit (ACU) pins prevents correct reading of the left middle-row side airbag circuit resistance.
  • 5Seat frame mounting bolt too long or interior trim clip crushing the wiring harness, causing the circuit to short to ground and blow the fuse, or physically severing the wire.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal. Wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Locate the module: Remove the left middle-row seat side trim panel or lower B-pillar trim panel. Locate the side airbag module (usually marked 'SIDE AIRBAG') and its dedicated yellow connector.
  • 3
    Visual inspection: Verify the connector is fully locked. Inspect the pins for oxidation, backed-out pins, or signs of water ingress. Confirm the wiring harness has no damage, crushing, or breaks.
  • 4
    Resistance measurement: Use a multimeter to measure the resistance at the airbag module terminal. The normal value is 2.0-3.0 Ω. An infinite reading indicates an internal open circuit in the module. Disconnect the module, measure the wiring harness terminal, and confirm the circuit between the SRS ECU and the module has continuity (<1 Ω) and proper insulation to ground and power supply.
  • 5
    Fault isolation: If the wiring harness is normal, connect a 3Ω substitute resistor to the harness connector. Clear the fault code. If the code does not reappear, replace the airbag module. If the fault persists, check the SRS control unit and wiring harness continuity.
  • 6
    Reset test: After repair, reconnect all components and the battery. Use the diagnostic tool to clear the fault code and perform the 'Airbag System Dynamic Test'. Confirm B172B1B does not return and the system status displays 'Normal'.
Real-World Cases
BYD DTC AI Analysis

BYD E2 seat wiring harness wear caused intermittent open circuit

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Vehicle: 2019 BYD E2, 32,000 km. Symptoms: Instrument cluster intermittently displayed the airbag warning light. Read fault code B172B1B (left mid-row side airbag not connected). Cleared the fault, but it reoccurred after driving on rough roads. Diagnosis: Removed the left front seat (the E2's "mid-row" side airbag is actually integrated into the left side of the driver's seat backrest). Found chafing between the seat frame and wiring harness mounting bracket. The harness insulation had worn through, with 3 internal copper wires broken, causing intermittent high contact resistance. Repair: Repaired the wiring (soldered and covered with heat shrink tubing), rerouted and secured the harness, fitted an anti-abrasion sleeve. Fault eliminated.
BYD DTC AI Analysis

Qin EV B-pillar connector water ingress and oxidation

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Vehicle: 2019 Qin EV, operated in a southern coastal city. Symptoms: After rain, the instrument cluster displayed an airbag fault. Scan tool retrieved DTC B172B1B. Diagnosis: Removed the lower left B-pillar trim panel. Found obvious water stains and green copper corrosion inside the side airbag connector (yellow plug). Pin oxidation caused excessive contact resistance (measured 12Ω). Cause: The B-pillar trim seal deteriorated, allowing rainwater to seep in and accumulate at the connector. Repair: Cleaned the connector pins, applied electrical contact protectant, and replaced the seal. Fault resolved.
BYD DTC AI Analysis

E3: Airbag connector not connected after seat replacement

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Vehicle: 2020 BYD E3, used. Symptom: Airbag warning light stayed on after purchase, fault code B172B1B. Diagnosis: Inspection found the left front seat (with side airbag) had been replaced, but the technician did not connect the airbag harness connector beneath the seat, or the connector did not click into place, causing a loose connection. Repair: Reseated the airbag connector and confirmed the locking tab engaged, cleared the fault code, and the system returned to normal.
BYD DTC AI Analysis

Airbag module internal squib open circuit

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Vehicle: 2019 BYD Qin EV, post-collision repair. Symptoms: After left-side collision repairs, the instrument cluster displayed an airbag system fault. Scanned DTC B172B1B. Diagnosis: Measured infinite resistance across the airbag module. The internal igniter was damaged during impact (airbag did not deploy), causing an open circuit. Alternatively, the airbag deployed in a previous accident and was replaced with a second-hand module that had already failed. Repair: Replaced the left second-row side airbag module with a new unit, secured it with special bolts, and configured the SRS system (some models require writing VIN or configuration code). Fault cleared.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 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.