AU
B178F-00

DTC B178F-00 indicates an abnormally low resistance connection (typically <1Ω) between the front passenger dual-stage frontal airbag Stage 2 squib circuit and body ground (GND) — Atto 8

Safety System

DTC B178F-00 indicates an abnormally low resistance connection (typically <1Ω) between the front passenger dual-stage frontal airbag Stage 2 squib circuit and body ground (GND).

The dual-stage airbag system achieves staged inflation via two independent igniters: the first stage triggers a small amount of gas, and the second stage triggers delayed supplemental inflation based on collision severity.

A short to ground causes: 1) The SRS control module to detect a circuit anomaly, illuminate the airbag warning lamp, and disable the system. 2) Unintended airbag deployment in extreme cases. 3) The second stage to fail to deploy normally during an actual collision, reducing protection effectiveness.

This fault involves a high-voltage ignition circuit and is a safety-critical fault.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Airbag wiring harness insulation worn: Long-term vibration or seat adjustment friction damages the wiring harness under the front passenger seat or inside the dashboard, wearing through the outer sheath and shorting it to the vehicle body metal.
  • 2Connector water ingress and corrosion: After vehicle wading or interior cleaning, the internal pins of the airbag connector (usually located under the center console or seat) become damp and oxidize, causing a Pin-to-Case short circuit.
  • 3SRS control module internal fault: A damaged internal driver chip or monitoring circuit in the airbag ECU falsely reports a short to ground (rule out by measuring resistance on the wiring harness side).
  • 4Improper repair work: Previous repairs to the center console, carpet, or seats left the wiring harness improperly secured, allowing the seat rail to pinch it or a screw to pierce it.
  • 5Airbag module internal igniter fault: Short circuit in the Stage 2 igniter body within the front passenger airbag module (less common, but possible).
What To Do
  • 1
    Safety preparation: Turn off the ignition, disconnect the 12V battery negative terminal, and wait at least 3 minutes for the SRS capacitor to fully discharge to prevent accidental airbag deployment.
  • 2
    Initial diagnosis: Connect the diagnostic tool to read all fault codes and freeze frame data. Confirm B178F-00 is a current fault (Active), not a historical fault. Record the vehicle mileage and ambient temperature.
  • 3
    Visual inspection: Remove the front passenger-side lower trim panel and check the SRS wiring harness connector (usually located behind the glove box or in the middle of the centre console) for signs of water ingress, terminal corrosion, or green oxidation.
  • 4
    Resistance measurement: Disconnect the airbag control module connector. Use a multimeter to measure the resistance between the pin corresponding to B178F (usually the front passenger airbag Stage 2 trigger wire) and body ground. Normal resistance is >1MΩ. Continuity indicates a short to ground.
  • 5
    Segment isolation: If resistance is abnormal, disconnect the front passenger airbag connector (located on the back of the airbag or inside the instrument panel). Measure resistance on the wiring harness side and the airbag side separately to determine whether the short circuit is in the wiring harness or the airbag assembly.
  • 6
    Wiring harness repair: If the wiring harness is worn, wrap the damaged area with insulating tape. Reroute the harness to avoid contact with metal edges. Replace the entire airbag wiring harness if necessary.
  • 7
    Component replacement: If the wiring harness is normal but the airbag assembly has an internal short circuit, replace the front passenger frontal airbag assembly (use genuine parts and record the inflator serial number).
  • 8
    System verification: Reconnect all connectors and the battery, clear the fault code, and perform the SRS system self-check (at Key ON, the warning light should illuminate for 6 seconds and then turn off). Perform a simulated crash test (use a dedicated resistor instead of the airbag; do not test the airbag directly).
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV450: Seat Adjustment Causing Wiring Harness Wear Case

✓
2018 Qin EV450 with 80,000 km: owner reported the airbag warning light stays on. Retrieved DTC B178F-00. Inspection found the SRS wiring harness beneath the front passenger seat rubbing against the seat rails during fore-aft adjustment. The insulation wore through, shorting the copper wires to ground. Repair: Repaired the harness, protected it with corrugated tubing, and repositioned the harness retaining clips to prevent interference throughout the full seat travel range. Fault resolved.
BYD DTC AI Analysis

Qin 100 airbag connector corrosion after water ingress

✓
2017 BYD Qin 100 developed a B178F-00 fault code after driving through heavy floodwater. Disassembly revealed deterioration of the airbag connector seal below the centre console; water ingress caused short circuits between pins and to earth. Cleaned the connector and treated it with electronic contact cleaner; insulation resistance returned to normal. Replace the connector seal and apply conductive grease for protection.
BYD DTC AI Analysis

Wiring harness unsecured after accident repair caused short circuit

✓
A Qin 80 with a previous front-end collision set DTC B178F-00 one month after the dealership replaced the passenger airbag. The technician had not secured the airbag harness properly; the dashboard metal bracket pinched it, causing a short to ground. Rerouted the harness and fitted a protective sleeve. Cleared the fault. After accident repairs, verify harness routing and securing.
BYD DTC AI Analysis

SRS Control Module False Positive Case

✓
2017 Qin EV300 intermittently threw DTC B178F-00, sometimes returning days after clearing. Airbag harness and assembly resistance measured normal (2.1Ω, insulation to ground >10MΩ). Diagnosed as SRS control module internal monitoring circuit fault. Replaced the airbag control module and recoded/matched; fault has not returned.
Other Atto 8 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.