AU
B178A

DTC B178A indicates the Stage 2 Passenger Front Airbag inflator circuit resistance measures 0 ohms (or near 0 ohms; normal range is 2 — Atto 8

Safety System

DTC B178A indicates the Stage 2 Passenger Front Airbag inflator circuit resistance measures 0 ohms (or near 0 ohms; normal range is 2.0–3.0 ohms). "Stage 2" refers to the secondary trigger circuit of a dual-stage airbag, providing greater deployment energy during a severe collision.

A resistance of 0 ohms indicates a hard short (short to ground or short to power), not an open circuit.

This fault triggers the SRS fail-safe mechanism: the control unit disables the passenger airbag Stage 2 deployment function, or disables the airbag entirely, and illuminates the instrument cluster SRS warning light.

During a collision, this causes insufficient passenger protection, posing a serious safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Airbag inflator internal short circuit: Manufacturing defects, long-term aging, moisture ingress, or previous accident damage causes the internal squib resistance to drop to 0 ohms, creating a hard short circuit.
  • 2Harness short to ground/power supply: Metal edges cut the instrument panel internal harness insulation, retaining screws compress and chafe the insulation, or water ingress corrodes the harness, causing a short to vehicle body ground or +12V power supply.
  • 3Connector terminal fault: Dedicated yellow front passenger airbag connector (usually located behind the glovebox or inside the dashboard) has bent terminals, backed-out pins, loose connections, or electrolyte corrosion from water ingress causing a short circuit between pins.
  • 4SRS ECU internal detection circuit fault: Damage to the airbag control unit's internal analog-to-digital (A/D) converter or detection chip causes incorrect resistance readings.
  • 5Improper repair procedures: Connecting or disconnecting the airbag connector with the battery connected, generating an arc, or directly measuring a live airbag circuit using a standard multimeter resistance setting, causing the internal squib to blow or short circuit.
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds (120 seconds for some models) to fully discharge the SRS backup capacitor and prevent accidental airbag deployment and personal injury during repair.
  • 2
    Fault status confirmation: Connect the diagnostic tool and read the B178A freeze frame data. Confirm whether the fault is current (Active) or history (History). Record the ambient temperature and vehicle status at the time of the fault. Check for other accompanying SRS fault codes (such as B1780, B1781, etc.).
  • 3
    Initial visual inspection: Remove the glove box or front passenger lower trim panel. Inspect the SRS-specific yellow wiring harness and connectors for obvious damage, scorching, or water ingress. Verify the airbag identification label is intact and confirm it is a genuine part.
  • 4
    Circuit isolation diagnosis: Disconnect the front passenger second-stage airbag module connector. Connect a dedicated airbag substitute resistor (2.7Ω/0.25W) to the wiring harness side. Connect the diagnostic tool and read the data stream. If the tool displays a normal resistance (2.0-3.0Ω), the fault is in the airbag module. If it still displays 0Ω, the fault is in the wiring harness or ECU.
  • 5
    Harness continuity check: Use a high-precision multimeter to measure the resistance to ground at both ends of the airbag harness (should be >1MΩ). Measure the continuity between both ends of the harness (should be <1Ω). Check for a short circuit to the +B power supply (voltage should be 0V with the ignition switch ON).
  • 6
    Airbag module inspection: If the wiring harness is normal, measure the resistance between the airbag module terminals. Normal resistance is 2.0-3.0 Ω. A reading of 0 Ω or <1 Ω indicates an internal short circuit in the module. Replace the entire front passenger airbag assembly (do not dismantle for repair).
  • 7
    Repair and replacement: Repair damaged wiring harnesses (use high-temperature insulating tape and corrugated conduit, maintaining clearance from the vehicle body). Replace short-circuited connectors or the airbag module. Use only genuine parts when replacing the airbag. On some models, perform the 'Airbag Replacement Record' or 'VIN Write' operation on the diagnostic tool.
  • 8
    System reset and verification: Reconnect all connectors and the battery. Clear the fault code using the diagnostic tool. Execute "SRS System Configuration" and "Acceleration Sensor Zero Point Calibration". Perform an ignition cycle self-check (IG ON → IG OFF three times). Confirm B178A does not reappear and the SRS warning lamp turns off.
Real-World Cases
BYD DTC AI Analysis

Water-damaged vehicle, passenger-side A-pillar wiring harness chafed and shorted

✓
A 2019 BYD Song MAX developed active fault code B178A after driving through heavy floodwater. Inspection revealed standing water under the front passenger carpet that hadn't drained. The wiring harness retaining clip inside the A-pillar had detached, causing the harness to chafe against the metal bracket until the insulation wore through and rainwater caused a short to ground. Repair action: Cleaned up the water and dried the interior thoroughly, replaced the damaged wiring harness section (approx. 40cm), re-secured the harness routing with cable ties, and ensured more than 20mm clearance from metal edges.
BYD DTC AI Analysis

Aftermarket airbag module internally shorted after accident repair

✓
2021 BYD Tang. After front-end collision repairs at a non-authorized shop, the SRS light stayed on following passenger airbag replacement. DTC B178A stored and would not clear. Harness side resistance measured normal, but the replacement airbag module (aftermarket second-hand unit) showed 0.3Ω across the terminals—indicating an internal short in the gas generator. Installed a new genuine airbag module; resistance returned to 2.4Ω and the fault cleared. Note: Airbag components must be genuine new parts; second-hand parts present serious safety hazards.
BYD DTC AI Analysis

Blocked AC drain caused connector corrosion and short circuit

✓
BYD Yuan EV owner reported the SRS warning light occasionally coming on. Scan tool showed DTC B178A as an intermittent fault. Inspection found the air conditioning evaporator drain hose blocked with leaves; condensate overflowed behind the glove box and oxidized the passenger airbag connector pins (yellow 2-pin plug), creating verdigris and a micro-short. Cleaned the connector terminals with electronic contact cleaner, applied conductive protective grease, cleared the drain hose, and wrapped the connector with waterproof tape.
BYD DTC AI Analysis

Dashboard frame pinched wiring harness after aftermarket large screen installation

✓
A Qin Pro logged code B178A one week after its centre screen was replaced at a third-party modification workshop. Removing the dashboard revealed the installer had failed to properly reposition the passenger airbag wiring harness. The instrument panel metal frame crushed the harness, fracturing the internal copper conductors and causing a short to earth. Technicians rerouted the wiring, fitted rubber protective sleeving, and repaired the damaged conductors to resolve the fault. Have qualified professionals perform any dashboard removal work, and verify SRS wiring harness integrity.
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.