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 — Atto 3

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 — Atto 3
  • 1Mechanical damage to the under-seat wiring harness: During fore/aft 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 track.
  • 2Airbag connector water ingress and corrosion: Poor sealing of the airbag wiring harness connector under the seat (usually a yellow plug) allows water entry during vehicle wading or interior cleaning. Oxidized internal terminals cause a short to ground.
  • 3Airbag module internal igniter short circuit: Insulation failure of the igniter resistance wire inside the airbag gas generator causes a direct short to the metal housing.
  • 4B-pillar trim panel pinching the wiring: During B-pillar repair or floor mat installation, trim panel clips crush and damage the wiring harness, causing a short to body ground.
  • 5Seat heater pad retrofit interference: fixing pins used during aftermarket seat heater pad installation pierce the airbag wiring harness and damage 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 the BYD dedicated diagnostic tool (VDS2000/ED400) to read the DTC freeze frame data. Record the vehicle status at the time of the fault (mileage, temperature, voltage). 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 infinite (OL). A reading of 0-5Ω indicates a short to ground in the circuit.
  • 4
    Segmented fault tracing: Gradually strip back the corrugated conduit from the airbag connector toward the ACU. Inspect the wiring harness closely at the seat slide rail mounting points and seat hinges. Check for damaged insulation or exposed copper wire. Perform a Wiggle Test to reproduce intermittent short circuits if necessary.
  • 5
    Connector inspection: Inspect the plug terminals for green corrosion, backed-out pins, or deformation. Clean with electrical contact cleaner and measure the insulation resistance between the terminals and to ground. Standard value: >10MΩ.
  • 6
    Module verification: If circuit measurement is normal (infinite), replace the airbag module and measure the new module resistance (2.0±0.3Ω) to confirm the fault is internal to the module.
  • 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 the battery. Use the diagnostic tool to perform 'SRS system self-check' and 'configuration verification'. Verify B172C changes to a history fault and clear the DTC. Finally, perform a vehicle crash simulation test (using a simulated load) to verify 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 Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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.