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 — Qin Plus

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 Qin Plus indicates a short circuit 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 function. Common causes include a shorted inflator or damaged wiring. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from 4 recorded cases.
4
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Internal short circuit in the airbag inflator: Manufacturing defects, long-term aging, moisture ingress, or previous accident damage reduces internal squib resistance to 0 ohms, creating a hard short circuit.
  • 2Wiring harness short to ground/power supply: Metal edges cut the internal dashboard wiring harness insulation, retaining screws pinch and chafe the insulation, or water ingress corrodes the harness, causing a short to body ground or the +12V power supply.
  • 3Connector terminal fault: Bent terminals, backed-out pins, loose connections, or a pin-to-pin short circuit caused by water ingress and electrolytic corrosion at the yellow dedicated front passenger airbag connector (usually located behind the glovebox or inside the dashboard).
  • 4SRS ECU internal detection circuit fault: Damage to the airbag control unit's internal analog-to-digital converter (A/D converter) or detection chip generates incorrect resistance readings.
  • 5Improper repair procedures: Disconnecting or connecting the airbag connector without disconnecting the battery, causing arcing; 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: Switch 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 if the fault is current (Active) or historical (History). Record the ambient temperature and vehicle status at the time of the fault. Check for other 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 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 and 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 airbag module is faulty. If it still shows 0Ω, the wiring harness or ECU is faulty.
  • 5
    Harness continuity check: Use a high-precision multimeter to measure resistance to ground at both ends of the airbag harness (must be >1 MΩ), measure continuity between both ends of the harness (must be <1 Ω), and check for a short circuit to +B power (voltage must be 0 V with the ignition switch ON).
  • 6
    Airbag module inspection: If the wiring harness is normal, measure the resistance between the airbag module pins. Normal resistance is 2.0-3.0 Ω. A reading of 0 Ω or <1 Ω indicates an internal short circuit. Replace the entire front passenger airbag assembly (do not disassemble or repair).
  • 7
    Repair and replacement: Repair damaged wiring harnesses using 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. For 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). Confirm 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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.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 Qin Plus →Qin Plus 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.