AU
B178A1A

DTC B178A1A indicates the SRS control module detects the Passenger Second Stage Front Airbag igniter circuit resistance at or near 0 ohms — Seal 6 EV

Safety System

DTC B178A1A indicates the SRS control module detects the Passenger Second Stage Front Airbag igniter circuit resistance at or near 0 ohms.

The second-stage airbag uses dual-stage ignition technology to control the deployment time interval based on collision severity.

A resistance of 0 ohms indicates a short circuit fault in the airbag igniter circuit (short between wires or short to ground), not an open circuit.

This fault causes the SRS system to enter degraded mode: the system disables this specific airbag circuit to prevent accidental deployment and illuminates the airbag warning light.

In a frontal collision, the passenger side may lose second-stage cushioning protection, deploying only the first-stage airbag or failing to deploy entirely, which significantly reduces occupant protection.

The B178A1A fault code on the Seal 6 EV indicates a short circuit in the passenger second-stage front airbag igniter circuit, with resistance near 0 ohms. This is a medium-severity SRS fault that disables the affected airbag circuit and turns on the airbag warning light. Common causes include an internal airbag igniter short or chafed dashboard wiring behind the glove box. 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 — Seal 6 EV
  • 1Front passenger airbag module internal igniter short circuit: Fused igniter bridge wire inside the airbag, or moisture in the pyrotechnic charge causing a sudden drop in resistance.
  • 2Internal dashboard wiring harness chafed and shorted: Vibration and friction damage the insulation on the Stage 2 airbag wiring harness (usually located on the dashboard crossmember or behind the glove box), causing a short circuit to the body metal.
  • 3Connector water ingress and corrosion: A/C condensate leakage or vehicle wading causes a short circuit between pins on the front passenger side floor wiring harness connector (e.g., CJ103 or near ground point G301).
  • 4SRS control module internal detection circuit fault: A damaged ECU internal sampling resistor or A/D conversion circuit triggers a false short circuit fault.
  • 5Modification or repair damage: Screws crush, drill bits pierce, or retaining clips pinch the wiring harness when installing a dashcam or audio system on the front passenger side, or during accident repairs.
What To Do
  • 1
    Safety preparation: Turn the vehicle OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS backup power supply.
  • 2
    Fault confirmation: Use a VDS or Launch diagnostic tool to read all DTCs. Confirm B178A1A is a current fault (Active), not a history fault. Check for accompanying fault codes (e.g., B178B - front passenger second-stage resistance too low).
  • 3
    Physical inspection: Remove the front passenger lower dashboard trim panel. Visually inspect the yellow SRS wiring harness corrugated conduit for damage or burn marks. Inspect the airbag connector (usually located behind the glove box or on the dashboard crossmember) for looseness or water ingress.
  • 4
    Airbag assembly inspection: Disconnect the front passenger second-stage airbag connector (use a shorting clip or anti-static measures). Use a digital multimeter to measure the resistance between the airbag-side terminals. The normal value is 2.0-5.0 Ω. If the reading is 0-1 Ω, replace the front passenger airbag module assembly.
  • 5
    Harness continuity check: Measure the resistance to ground of the circuit between the SRS ECU-side harness connector (with the ECU disconnected) and the airbag connector. The normal value is infinite. If the reading is 0 Ω or a low resistance, locate the short circuit in the harness. Inspect the dashboard bracket pass-through hole and interference points between the floor harness and the vehicle body.
  • 6
    ECU verification: If the wiring harness and airbag are normal, measure the resistance between the corresponding pins of the SRS ECU connector. If the resistance remains 0Ω, this indicates an internal ECU fault. Replace the airbag control module.
  • 7
    Repair and verification: After repairing or replacing the faulty component, reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform the 'SRS system self-check' and 'configuration write' (if replacing the ECU). Confirm the fault code does not reappear and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

BYD Qin EV Passenger Airbag Short Circuit After Wading

✓
A 2019 BYD Qin EV presented with a constant airbag warning light after driving through deep floodwater. Scanned DTC B178A1A. Removed the interior trim and found the passenger-side floor wiring harness connector CJ103 had accumulated condensation due to a blocked air conditioning drain tube, causing internal pin oxidation and shorting. Cleaned the connector and applied conductive adhesive, replaced the wiring harness seal, and cleared the fault. Also check the air conditioning drain tube for blockages.
BYD DTC AI Analysis

BYD E2 short circuit from pinched wiring harness after accident repair

✓
2020 E2 set DTC B178A1A after front collision repair. Inspection found the instrument panel crossmember mounting bolts installed during the accident repair were too long, penetrating the passenger side second-stage airbag harness corrugated tubing and causing the wire cores to short to body. Replaced the damaged harness section (Part No.: BYD-BCM-SRS-05) and adjusted the bolt length. Fault resolved. Note: Accident repairs must strictly follow repair manual torque specifications.
BYD DTC AI Analysis

BYD E3 airbag module internal igniter aging

✓
An E3 with 80,000 km exhibited intermittent B178A1A faults in humid conditions. With the harness disconnected, the airbag module resistance read normal at ambient temperature but dropped sharply to 0.5 Ω when heated to 60°C. Diagnosed as thermal breakdown of the aged internal igniter insulation. Replaced the passenger airbag assembly and dashboard trim panel (integrated design). Fault resolved.
BYD DTC AI Analysis

Aftermarket passenger-side glove box modification damaged the wiring harness

✓
The owner fitted ambient lighting to the passenger glovebox, drilling holes that damaged the second-stage airbag wiring harness. DTC B178A1A appeared intermittently. Inspection revealed the drill bit had scratched the wiring behind the glovebox, exposing the copper core which contacted the metal dashboard bracket. Cut out the damaged section, soldered in an extension wire, and insulated with double-layer heat shrink tubing to repair the harness. Restored the wiring to its original routing position. Fault cleared. Recommend prohibiting non-professional modifications involving SRS circuits.
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.