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B178A1A

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

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 Qin Plus indicates a short circuit in the passenger second-stage front airbag igniter circuit, with resistance at or near 0 ohms. This is a medium-severity SRS fault that disables the affected airbag circuit and turns on the airbag warning light, reducing passenger protection in a crash. Common causes include a shorted airbag igniter or damaged dashboard wiring. 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 — Qin Plus
  • 1Front passenger side airbag module internal igniter short circuit: Fused airbag internal igniter bridge wire or damp pyrotechnic charge causing a sharp drop in resistance.
  • 2Instrument panel internal wiring harness wear and short circuit: Vibration and friction damage the insulation of the stage 2 airbag wiring harness (usually located on the instrument panel crossmember or behind the glove box), shorting it 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: A damaged ECU internal sampling resistor or A/D conversion circuit triggers a false short circuit fault.
  • 5Modification or repair damage: When installing a dash cam or audio system on the front passenger side, or during accident repairs, screws crush, drilling pierces, or retaining clips pinch the wiring harness.
What To Do
  • 1
    Safety preparation: Turn the vehicle OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds for the SRS backup power supply to fully discharge.
  • 2
    Fault confirmation: Use a VDS or Launch diagnostic tool to read all DTCs. Confirm B178A1A is a current (Active) fault, 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 scorch marks. Inspect the airbag connector (typically located behind the glove box or on the dashboard crossmember) for looseness or water ingress.
  • 4
    Airbag module inspection: Disconnect the front passenger second-stage airbag connector (use a shorting clip or anti-static precautions). Use a digital multimeter to measure the resistance between the airbag-side terminals. Standard resistance is 2.0-5.0 Ω. If the reading is 0-1 Ω, replace the front passenger airbag module assembly.
  • 5
    Harness continuity check: Disconnect the SRS ECU. Measure the resistance to ground of the circuit between the SRS ECU harness connector and the airbag connector. Normal value: Infinite. If the reading is 0 Ω or low, locate the short circuit in the harness. Focus on the dashboard bracket wiring pass-through hole and floor harness-to-body interference points.
  • 6
    ECU verification: If both 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. Clear the fault code using the diagnostic tool. 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 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.