AU
B172D

DTC B172D indicates the Supplemental Restraint System (SRS) detects a short to vehicle power (B+) in the Left Second Row Side Airbag Module ignition circuit — Seal U

Safety System

DTC B172D indicates the Supplemental Restraint System (SRS) detects a short to vehicle power (B+) in the Left Second Row Side Airbag Module ignition circuit.

During self-diagnostics or continuous monitoring, the Airbag Control Unit (ACU) detects an abnormally high voltage (near 12V battery voltage) in the left second row side airbag deployment circuit.

This exceeds the normal monitoring range (typically a low voltage or specific resistance value).

This short to power forces the airbag system into fail-safe mode, disables the affected airbag to prevent accidental deployment, and illuminates the airbag fault warning lamp.

As the second row side airbag is part of the passenger restraint system, this fault constitutes a severe safety risk.

The short circuit can prevent the airbag from deploying during a collision or, in extreme cases, trigger unintended deployment.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Worn or pinched harness under the seat: Frequent fore-and-aft adjustment of the left middle-row seat or passengers stepping on the harness damages the side airbag harness insulation, causing contact with the power wire.
  • 2Airbag module internal short circuit: The igniter bridge wire inside the left middle-row side airbag assembly shorts to the housing or power terminal, usually due to manufacturing defects or water ingress.
  • 3Connector terminal misalignment: Backed-out or bridged terminals in the yellow airbag connector under the seat (usually near the seat rail) cause a short circuit between the ignition circuit terminal and the power supply terminal.
  • 4Internal ACU fault: A fault in the airbag control unit internal drive circuit or monitoring circuit generates a false short-to-power diagnosis.
  • 5Improper repair procedures: A tool pierced the wiring harness during recent seat removal and installation, interior trim panel removal, or floor cleaning, or an incompletely locked connector caused the terminal to contact the power wire.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS system capacitor. Wear an anti-static wrist strap and disable the vehicle high-voltage system (for hybrid/pure electric models).
  • 2
    Fault Confirmation and Freeze Frame Analysis: Use the BYD dedicated diagnostic tool (VDS or ED400) to read the fault code. Confirm B172D is a current fault (Active). Record the voltage values in the freeze frame data and the vehicle status at the time of the fault.
  • 3
    Wiring visual inspection: Remove the left middle-row seat and inspect the airbag wiring harness inside the yellow harness sleeve under the seat for abrasion, cuts, or burning. Focus on the seat rail mounting points and the wiring harness pass-through grommet.
  • 4
    Connector inspection: Disconnect the airbag connector under the seat (usually marked as DAB or SAB related). Inspect the terminals for corrosion, signs of water ingress, or short circuits between terminals. Measure the resistance between the connector terminals (normal: 2.0-5.0 Ω).
  • 5
    Short circuit location: Use a multimeter to measure continuity between the airbag harness side (not the module side) and the positive power supply. If continuity exists, inspect the harness in sections along its routing to locate the short circuit to the vehicle power wire (constant power or ignition power).
  • 6
    Module isolation test: Disconnect the airbag module connector and connect a dedicated airbag simulator (2Ω substitute resistor) to the circuit. Clear the fault code. If the fault disappears, the airbag module has an internal short circuit. Replace the left middle-row side airbag assembly.
  • 7
    Wiring harness repair or replacement: If the wiring harness is damaged, repair it using high-temperature tape and corrugated conduit. Maintain sufficient clearance from the power cable. If the wiring harness is severely damaged, replace the entire seat wiring harness assembly.
  • 8
    System verification: After repair, reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code and perform the SRS system self-check (usually requires cycling the ignition switch ON/OFF 3 times). Confirm B172D does not return and the airbag warning lamp turns off. Finally, perform a full-range seat fore/aft adjustment test to verify the wiring harness has no interference.
Real-World Cases
BYD DTC AI Analysis

Worn wiring harness for the left second-row side airbag caused B172D on a Song MAX.

✓
The vehicle is a 2019 Song MAX with 32,000 km. The airbag warning light stays on. Diagnostic scan tool retrieved DTC B172D (current fault). Inspection found the protective sleeve on the wiring harness beneath the second-row left seat was damaged. The harness had chafed at the seat slide rail mounting point, wearing through the internal wire insulation. The exposed wire contacted the seat heater power wire (constant power), causing a short circuit. Repaired the damaged harness, rerouted the wiring away from the seat slide rail travel path, and added an abrasion-resistant protective sleeve. Fault resolved.
BYD DTC AI Analysis

Water ingress and corrosion in the left middle-row side airbag connector on the Tang DM.

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2021 BYD Tang DM. Airbag warning light came on after the vehicle waded through water. Diagnosis revealed fault code B172D. Removed the carpet, disconnected and inspected the airbag connector under the left second-row seat. Found water stains and green terminal corrosion causing a short between terminals. Water entered through the door sill trim while wading, and the connector seal had deteriorated. Repair: cleaned and dried the connector, sprayed electronic contact cleaner, replaced the seal, and cleared the fault code. System returned to normal.
BYD DTC AI Analysis

Yuan EV side airbag module internal short circuit false alarm

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2019 BYD Yuan EV, no prior accidents. Airbag warning light suddenly came on; VDS scan showed B172D. Checked wiring harness and connectors – no damage or abnormalities. Disconnected the airbag module and tested with a simulator; fault code cleared. Reconnected the module and the fault reappeared. Diagnosed as a short to power in the igniter inside the left middle-row side airbag assembly. Replaced the OEM left middle-row side airbag module (part number matched to VIN). Programmed and configured the system; fault completely eliminated.
BYD DTC AI Analysis

Qin Pro wiring harness pinched and shorted after seat modification

✓
2020 BYD Qin petrol version. Owner self-installed a seat ventilation system; airbag warning light came on. Diagnosed DTC B172D. Inspection found the wiring harness was not secured properly during the modification—the new fan power wire and side airbag harness were pinched at the seat frame, causing insulation damage and a short circuit. Repair: separated and re-secured the harness using OEM harness clips to ensure clearance, and repaired the damaged insulation. 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.