AU
B172C

DTC B172C indicates the Airbag Control Unit (ACU) detects an abnormally low-resistance path (typically <1Ω) between the left second-row side airbag squib circuit and body ground — Seal U

Safety System

DTC B172C indicates the Airbag Control Unit (ACU) detects an abnormally low-resistance path (typically <1Ω) between the left second-row side airbag squib circuit and body ground.

In the SRS system, normal airbag squib resistance is approximately 2.0-3.0Ω.

When the ACU detects a short to ground in the circuit, it registers a severe fault and triggers protection mode: immediately disconnecting power to the airbag circuit, illuminating the SRS warning lamp, and disabling left second-row side airbag deployment.

This fault prevents the side airbag from inflating normally to protect the occupant during a collision, or in extreme cases, risks unintended deployment due to intermittent wiring contact.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Mechanical damage to the wiring harness under the seat: During forward/backward adjustment of the left middle-row seat, the seat frame rubs against the floor wiring harness, damaging the insulation and causing the wire to contact the metal seat rail.
  • 2Airbag connector water ingress and corrosion: Water enters the poorly sealed under-seat airbag wiring harness connector (usually a yellow plug) when driving through water or cleaning the interior. This oxidizes the internal terminals, causing a short to ground.
  • 3Airbag module internal igniter short circuit: The igniter resistance wire insulation inside the airbag inflator fails and shorts directly to the metal housing.
  • 4B-pillar trim panel pinching wiring: Trim panel clips crush and damage the wiring harness during B-pillar repair or floor mat installation, causing a short to body ground.
  • 5Seat heater pad retrofit interference: retaining pins used during aftermarket seat heater pad installation pierce the airbag wiring harness, damaging the insulation.
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
    Diagnostic confirmation: Use BYD dedicated diagnostic tool (VDS2000/ED400) to read DTC freeze frame data. Record vehicle status at the time of the fault (mileage, temperature, voltage) and confirm B172C is a current fault, not a history fault.
  • 3
    Circuit isolation test: Disconnect the left middle-row side airbag module connector (located under the seat, yellow 2-pin plug). Use a multimeter to measure the resistance to ground at both terminals of the harness-side connector. Normal resistance is infinity (OL). A reading of 0-5Ω indicates a short to ground in the circuit.
  • 4
    Section-by-section fault isolation: Gradually strip back the corrugated conduit from the airbag connector toward the ACU. Inspect the wiring harness, focusing on the seat slide rail mounting points and seat hinges. Check for damaged insulation or exposed copper wire. If necessary, perform a Wiggle Test to reproduce intermittent short circuits.
  • 5
    Connector inspection: Inspect plug terminals for green corrosion, backed-out pins, or terminal deformation. Clean with electrical contact cleaner and measure insulation resistance between terminals and to ground. Standard value: >10MΩ.
  • 6
    Module verification: If the circuit measurement is normal (infinite resistance), replace the airbag module and measure the new module resistance (2.0±0.3Ω) to confirm an internal module fault.
  • 7
    Repair and Verification: Repair the damaged wiring harness (use dual-wall heat-shrink tubing or replace the entire harness section). Reconnect all connectors and reconnect the battery. Use the diagnostic tool to perform 'SRS system self-check' and 'configuration verification'. Confirm DTC B172C changes to a history fault and clears successfully. Finally, perform an on-vehicle collision simulation test (using a simulated load) to confirm normal system operation.
Real-World Cases
BYD DTC AI Analysis

Song MAX left second-row seat rail wiring harness chafing case

✓
2019 BYD Song MAX, 28,000 km, instrument panel SRS light stays on. Diagnostic scan read current fault B172C. Disassembly inspection found the left middle-row seat slide rail rear mounting bolt had crushed the wiring harness. Repeated friction from fore-aft seat movement wore through the insulation on two wires, allowing the copper strands to contact the seat steel frame and short to ground. Rerouted the harness clear of the slide rail travel path, wrapped it with cloth tape, secured it to the seat base wiring bracket, and replaced the damaged wires. Fault cleared.
BYD DTC AI Analysis

Tang DM airbag connector corrosion after water wading

✓
2021 Tang DM. After wading through about 30cm of water, the dash showed a safety system fault. Scan found codes B172C and B172D for both middle-row side airbags. Pulled the seat airbag connectors; inside were clear water stains and green corrosion. Insulation resistance between terminals measured only 0.8Ω. Root cause: degraded connector seals let moisture in during wading, shorting terminals together and to ground. Repair: replaced both side airbag harness connectors (waterproof seals), cleaned airbag module terminals, applied conductive protectant, reassembled. Fault has not returned.
BYD DTC AI Analysis

Aftermarket seat heating modification causes short circuit in Yuan EV

✓
2019 BYD Yuan EV. SRS warning light came on the day after front seat heaters were installed at an auto parts market. Retrieved DTC B172C. Inspection found that a metal staple used to secure the heating pad had pierced the side airbag harness corrugated tubing, puncturing the igniter wire and contacting the seat frame. Repair: removed the unauthorized staples, replaced the damaged airbag harness assembly (igniter wiring cannot be spliced), and restored to factory condition. System normal.
BYD DTC AI Analysis

Qin Pro airbag module internal short circuit case

✓
2020 BYD Qin petrol, no accident or modification history. SRS warning lamp illuminated intermittently. Scan tool showed B172C intermittent fault. Measured under-seat wiring harness resistance to ground: normal. Wiggle test produced no change. Disconnected the airbag module and measured 0.2Ω between module terminals (abnormally low). Diagnosed an internal igniter short circuit in the airbag gas generator. Replaced the left middle-row side airbag assembly (including gas generator and airbag cushion). Performed ACU configuration write. Resolved the fault.
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.