AU
B172B

DTC B172B indicates the SRS (airbag) control unit detects an open circuit or disconnected condition in the left middle-row side airbag module (typically located inside the left B-pillar trim or on the side of the left rear seat) — Seal U

Safety System

DTC B172B indicates the SRS (airbag) control unit detects an open circuit or disconnected condition in the left middle-row side airbag module (typically located inside the left B-pillar trim or on the side of the left rear seat).

An abnormal resistance value in the circuit between the control unit and the airbag squib (typically infinite or outside the normal 2-3Ω range) causes the system to determine the airbag module is physically disconnected or internally open.

This fault prevents the left middle-row side airbag from deploying during a side-impact collision.

The SRS system may also enter a degraded protection mode, continuously illuminate the instrument panel airbag warning light, and restrict seat belt pretensioner function on some models.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose connection, backed-out terminals, or poor contact at the left middle row side airbag module connector (commonly due to connector oxidation after water ingress, or an improperly seated connector following B-pillar trim panel removal and installation)
  • 2Airbag wiring harness open or shorted from chafing (especially the yellow SRS harness under the seat or at the B-pillar, where fore-and-aft seat adjustment or frequent door opening and closing causes harness fatigue and breakage).
  • 3Internal open circuit in the left middle row side airbag module (gas generator igniter aging or internal solder joint detachment, causing abnormal resistance)
  • 4SRS control unit internal detection circuit fault (damaged control unit interface chip falsely reporting a left side airbag open circuit)
  • 5Airbag module connector locking mechanism failure after a collision or severe vibration, causing a poor connection.
What To Do
  • 1
    Safe power isolation: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 3 minutes to fully discharge the SRS capacitor and prevent accidental airbag deployment during repair.
  • 2
    Visual inspection: Remove the left B-pillar lower trim panel or left rear seat side cover. Check the left middle-row side airbag module (yellow connector) for looseness, water ingress, corrosion, or backed-out pins. Clean the connector and reconnect it securely. Confirm the locking tab is fully engaged.
  • 3
    Circuit continuity test: Use a multimeter to measure wiring harness continuity between the airbag module connector and the SRS control unit (usually under the centre console or centre tunnel). Check for open circuits, short circuits, or shorts to ground. Focus on harness bend points at the B-pillar hinge and under the seat.
  • 4
    Resistance measurement: Use a dedicated airbag simulator or a multimeter (only after powering off the system and disconnecting the module connector) to measure the airbag module resistance. Normal resistance is 2.0-3.0Ω. An infinite or 0Ω reading indicates an internal module fault.
  • 5
    Cross-check: Swap the left middle-row side airbag module with the right side module (or connect a known-good simulator). Clear the fault code and power on again. Observe if the fault code follows the module to determine whether the fault is in the module or the wiring harness/control unit.
  • 6
    Control unit inspection: If the wiring harness and module are normal, check the SRS control unit connector pins for oxidation or deformation. If necessary, measure the control unit internal interface circuit to confirm an internal control unit fault.
  • 7
    System reset verification: After repair, reconnect all components and the battery. Use VDS or a dedicated diagnostic tool to clear the fault code. Perform the SRS system self-check (usually requires 3 ignition cycles). Verify the instrument panel warning light turns off and the fault code does not return.
Real-World Cases
BYD DTC AI Analysis

BYD Tang B172B fault: Oxidised left middle row side airbag connector caused open circuit

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Symptoms: 2021 BYD Tang, airbag warning light stays on. DTC B172B stored (left second-row side airbag module disconnected). No collision history. Diagnosis: Inspected the airbag connector below the left B-pillar. Found green oxide inside with poor pin contact. Wiring harness continuity normal. Airbag module resistance 2.3Ω (normal). Repair: Cleaned connector pins with electronic cleaner, applied conductive grease, and reconnected. Cleared fault code; no recurrence. Analysis: Vehicle previously went through water. Moisture entered the B-pillar connector, causing oxidized high-resistance contact.
Original source ↗
BYD DTC AI Analysis

BYD Song MAX seat wiring harness chafing caused B172B

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Symptoms: 2019 Song MAX, airbag warning light illuminated intermittently while driving, diagnostic tool showed intermittent B172B fault. Diagnosis: Inspection found the SRS wiring harness beneath the left second-row seat interfering with the seat rail, with worn insulation causing the internal wires to intermittently open circuit. Repair: Repaired the damaged harness, wrapped it with corrugated tubing, and re-secured the harness routing to ensure safe clearance from the seat mechanism. Fault cleared.
Original source ↗
BYD DTC AI Analysis

BYD Qin Pro Left Second-Row Side Airbag Module Internal Fault Replacement Case

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Symptoms: SRS warning light stays on after starting the BYD Qin Pro. Fault code B172B will not clear. Diagnosis: Measured infinite resistance (open circuit) across left middle row side airbag module. Wiring harness continuity and connector contacts were good. Diagnosed an open circuit in the airbag module's gas generator igniter. Resolution: Replaced left middle row side airbag module assembly (transport safety precautions observed for new part). Used diagnostic scan tool to configure and match the system, then ran airbag self-test. Fault resolved.
Original source ↗
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.