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 U

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.

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Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Front passenger side airbag module internal igniter short circuit: A fused igniter bridge wire inside the airbag or damp propellant causes a sudden drop in resistance.
  • 2Dashboard internal wiring harness wear and short circuit: Vibration and friction damage the insulation of the stage 2 airbag wiring harness (usually located on the dashboard crossmember or behind the glove box), causing a short circuit to body metal.
  • 3Connector water ingress and corrosion: A/C condensate leakage or vehicle wading causes a short circuit between pins in the front passenger side floor wiring harness connector (such as CJ103 or near ground point G301).
  • 4SRS control module internal detection circuit fault: Damage to the ECU internal sampling resistor or A/D conversion circuit triggers a false short circuit fault.
  • 5Modification or repair damage: Screws crushing, drill bits piercing, or retaining clips pinching the wiring harness during accident repairs or installation of a dash cam or audio system on the front passenger side.
What To Do
  • 1
    Safety Preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds for the SRS backup power supply to discharge fully.
  • 2
    Fault confirmation: Use VDS or a Launch diagnostic tool to read the complete DTCs. Confirm B178A1A is a current fault (Active), not a historical fault. Check for accompanying fault codes (such as 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 scorch 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 pins. 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 (ECU disconnected) and the airbag connector. The resistance should be infinite. If the resistance is 0 Ω or low, locate the short circuit in the harness. Focus on the dashboard bracket pass-through hole and areas where the floor harness contacts the 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 0 Ω, 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 return 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 U 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.