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B178A

DTC B178A indicates the Stage 2 Passenger Front Airbag inflator circuit resistance measures 0 ohms (or near 0 ohms; normal range is 2 — Seal 6 EV

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.

The B178A fault code on the Seal 6 EV indicates a hard short in the Stage 2 passenger front airbag inflator circuit, with resistance near 0 ohms instead of the normal 2.0–3.0 ohms. Severity is medium because the SRS may disable part or all of the passenger airbag, reducing crash protection. Common causes include a shorted inflator or damaged wiring harness. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Seal 6 EV
  • 1Airbag inflator internal short circuit: Manufacturing defects, long-term aging, moisture ingress, or previous accident damage cause the internal squib resistance to drop to 0 ohms, creating a hard short circuit.
  • 2Wiring harness short to ground/power supply: Metal edges cut the wiring harness insulation inside the instrument panel, retaining screws pinch and chafe the harness, or water ingress corrodes the harness, causing a short to body ground or the +12V power supply.
  • 3Connector terminal fault: The dedicated yellow front passenger airbag connector (usually located behind the glove box or inside the dashboard) has bent terminals, backed-out pins, loose connections, or pin-to-pin short circuits caused by electrolytic corrosion after water ingress.
  • 4SRS ECU internal detection circuit fault: A damaged internal analog-to-digital (A/D) converter or detection chip in the airbag control unit produces incorrect resistance readings.
  • 5Improper repair procedures: Connecting or disconnecting the airbag connector without disconnecting the battery (generating an arc), or directly measuring a live airbag circuit using a standard multimeter resistance setting, causes the internal squib to blow or short-circuit.
What To Do
  • 1
    Safety preparation: Turn the vehicle 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 to read the B178A freeze frame data. Determine if the fault is Active or History. Record the ambient temperature and vehicle status at the time of the fault. Check for accompanying SRS fault codes (e.g., B1780, B1781).
  • 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 signs of 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 data stream shows a normal resistance (2.0-3.0Ω), the fault is in the airbag module. If it still reads 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), and 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. Standard resistance is 2.0-3.0 Ω. A reading of 0 Ω or <1 Ω confirms an internal module short circuit. Replace the entire front passenger airbag assembly (do not dismantle for repair).
  • 7
    Repair and replacement: Repair the damaged wiring harness (use high-temperature insulating tape and corrugated conduit; maintain clearance from the vehicle body). Replace short-circuited connectors or the airbag module. Use only genuine parts to replace the airbag. On some models, execute 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). Verify B178A does not return and the SRS warning light 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 Seal 6 EV 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.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV 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.