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 8

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.

The B172C fault code on the Atto 8 indicates a high-severity SRS fault where the left second-row side airbag circuit is shorted to ground. This can disable that airbag and turn on the SRS warning light. Common causes include damaged under-seat wiring from seat movement or water/corrosion in the yellow airbag connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 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 rail.
  • 2Airbag connector water ingress and corrosion: Poor sealing of the under-seat airbag wiring harness connector (usually a yellow plug) allows water entry during vehicle wading or interior cleaning, oxidizing the internal terminals and causing a short to ground.
  • 3Airbag module internal igniter short circuit: Igniter resistance wire insulation inside the airbag gas generator fails, shorting directly to the metal housing.
  • 4B-pillar trim panel pinching wiring: When repairing the B-pillar or installing floor mats, trim panel clips crush and damage the wiring harness, causing a short to body ground.
  • 5Seat heater pad retrofit interference: during aftermarket seat heater pad installation, fixing pins 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) and confirm B172C is a current DTC, not a history DTC.
  • 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 of both terminals on the harness-side connector. Normal resistance is infinite (OL). A reading of 0-5Ω confirms a short to ground in the circuit.
  • 4
    Section-by-section fault tracing: Starting from the airbag connector, gradually open the corrugated conduit toward the ACU. Focus inspection on the wiring harness at the seat slide rail mounting points and seat hinges. Check for damaged insulation or exposed copper strands. If necessary, perform a Wiggle Test to reproduce the intermittent short circuit.
  • 5
    Connector inspection: Inspect plug terminals for green corrosion, backed-out pins, or deformation. Clean with electrical contact cleaner and measure insulation resistance between terminals and to ground. Standard value: >10MΩ.
  • 6
    Module verification: If the circuit measures normal (infinite resistance), replace the airbag module and measure the new module resistance (2.0 ± 0.3 Ω) 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). Reconnect all connectors and the battery. Use the diagnostic tool to perform 'SRS system self-check' and 'configuration verification'. Verify B172C becomes a history fault and clears successfully. Finally, perform a vehicle collision 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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω). This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module. Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module. This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision. The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.
Common problems reported for Atto 8 →Atto 8 overview →
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.