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 — Seal U

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 U indicates a medium-severity SRS fault where the Stage 2 passenger front airbag inflator circuit has near-zero resistance, meaning a hard short circuit. This can be caused by an internal airbag inflator short or damaged wiring shorting to ground or power. The passenger airbag may be disabled, reducing crash protection. 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 U
  • 1Internal short circuit in the airbag inflator: 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.
  • 2Wiring harness short to ground/power supply: Metal edges cut the internal dashboard wiring harness insulation, retaining screws pinch and chafe the harness, or water ingress corrodes the wiring, causing a short to vehicle body ground or +12V power supply.
  • 3Connector terminal fault: Front passenger airbag yellow dedicated connector (usually located behind the glove box or inside the dashboard) has bent terminals, backed-out pins, loose connections, or water ingress causing electrolyte corrosion and a short circuit between pins.
  • 4SRS ECU internal detection circuit fault: A damaged internal analog-to-digital converter (A/D converter) or detection chip in the airbag control unit generates incorrect resistance readings.
  • 5Improper repair procedures: Generating an arc by connecting or disconnecting the airbag connector without disconnecting the battery, or directly measuring a live airbag circuit using the resistance setting of a standard multimeter, causes the internal squib to blow or short-circuit.
What To Do
  • 1
    Safety preparation: Switch 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. This prevents 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 history (History). Record the ambient temperature and vehicle status at the time of the fault. Check for 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 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 special airbag substitute resistor (2.7Ω/0.25W) to the wiring harness side and connect the diagnostic tool to read the data stream. If the diagnostic 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 continuity between both ends of the harness (should be <1Ω), and check for a short circuit to the +B power supply (voltage should measure 0V with the ignition switch ON).
  • 6
    Airbag module inspection: If the wiring harness is normal, measure the resistance between the airbag module pins. Standard resistance is 2.0-3.0 Ω. If the reading is 0 Ω or <1 Ω, the module has an internal short circuit. Replace the entire front passenger airbag assembly (do not dismantle for 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 airbag modules. Use genuine parts for airbag replacement. For some models, execute the 'Airbag Replacement Record' or 'VIN Write' procedure using the diagnostic tool.
  • 8
    System Reset and Verification: Reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Execute 'SRS System Configuration' and 'Acceleration Sensor Zero Point Calibration'. Perform an ignition cycle self-check (IG ON → IG OFF three times). Verify DTC 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 U Fault Codes
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.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.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.