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) — Seal U

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.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Airbag wiring harness connector under the seat or inside the B-pillar trim panel is loose, disconnected, or making poor contact. This commonly occurs after frequent seat adjustment or failing to securely clip the B-pillar trim panel after removal and installation.
  • 2Wiring harness fatigue fracture, particularly harnesses routed through the seat frame or near door hinges, where long-term bending and pinching break the internal copper strands while the outer insulation remains intact.
  • 3Airbag module internal open circuit; abnormal internal squib resistance (open circuit or excessive resistance). Module aging, previous moisture exposure, or electrostatic discharge usually causes this.
  • 4Corrosion or poor contact at the SRS control unit (ACU) pins prevents the ACU from correctly reading the circuit resistance of the left second-row side airbag.
  • 5Excessively long seat frame mounting bolt or interior trim panel clip crushes the wiring harness, causing a short to ground and blowing the fuse, or physically severing the wiring.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and 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, then 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, and confirm the wiring harness has no damage, crushing, or breaks.
  • 4
    Resistance measurement: Use a multimeter to measure the airbag module terminal resistance. The normal value is 2.0-3.0 Ω; an infinite reading indicates an internal module open circuit. Disconnect the module and measure the wiring harness terminal. Verify the circuit between the SRS ECU and the module has continuity (<1 Ω) and proper insulation to ground and power.
  • 5
    Fault isolation: If the wiring harness is normal, connect a 3Ω substitute resistor to the wiring harness end. If the fault code does not return after clearing, 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 connect the battery. Use the diagnostic tool to clear the fault code and perform the 'Airbag System Dynamic Test'. Confirm B172B1B no longer appears 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 Seal U 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.