AU
B178C-00

B178C-00 indicates the resistance of the front passenger-side Stage 2 frontal airbag ignition circuit falls outside the normal range set by the SRS ECU (typically 1 — Qin Plus

Safety System

B178C-00 indicates the resistance of the front passenger-side Stage 2 frontal airbag ignition circuit falls outside the normal range set by the SRS ECU (typically 1.5–3.0 Ω).

In the BYD dual-stage airbag system, the Stage 2 gas generator provides higher deployment power during a severe collision.

High resistance indicates excessive impedance or a potential open circuit in the ignition circuit.

Potential causes include an internal open circuit in the airbag squib, poor contact at the wiring harness connector, or a clock spring fault.

This safety system hard fault forces the SRS ECU into fail-safe mode, disabling deployment of this airbag stage and severely reducing front passenger protection during a collision.

Repair immediately.

The B178C-00 fault code on the Qin Plus indicates the front passenger Stage 2 airbag ignition circuit resistance is outside the normal range. This is a medium-severity SRS fault that can disable part of the passenger airbag deployment. Common causes include a faulty airbag igniter or loose, corroded, or water-damaged wiring connectors. The recommended fix is to 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
  • 1Aging, moisture ingress, or an internal open circuit in the front passenger airbag module (PAB) second-stage igniter causes resistance to exceed 3.0 Ω.
  • 2The airbag wiring harness connector (usually located under the center console, behind the glove box, or at the airbag module interface) is loose, oxidized, has water ingress, or has backed-out pins, causing increased contact resistance.
  • 3Broken wire or poor contact in the second-stage circuit inside the clock spring (spiral cable), common on vehicles after turning the steering wheel beyond its end stop. (Note: On some models, the front passenger airbag also routes through the clock spring or a similar transfer connector.)
  • 4Improper repair after a collision, such as failing to tighten the connector to the specified torque during previous airbag removal and installation, or using non-genuine parts that cause an impedance mismatch.
  • 5Internal airbag ECU A/D sampling circuit fault or software calibration error causing a false abnormal resistance report (rare but requires inspection)
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 3 minutes for the SRS backup power supply to discharge fully. Wear an anti-static wrist strap. Do not use radio equipment near the airbag assembly.
  • 2
    Fault Confirmation and Freeze Frame Analysis: Use the BYD VDS2000/ED400 diagnostic tool to read DTCs. Confirm B178C-00 is a current fault (Active). Record the resistance reading and ambient temperature from the freeze frame data. Check for accompanying fault codes (e.g., B178B, B178D).
  • 3
    Wiring harness connector inspection: Verify the front passenger airbag module yellow wiring harness connector (usually located on the right side of the center console or behind the glove box) is securely locked. Verify the CPA (Connector Position Assurance) clip is fully seated. Inspect the terminals for green oxidation, burn marks, or backed-out pins. Clean with electrical contact cleaner if necessary.
  • 4
    Airbag module resistance measurement: Remove the front passenger airbag module (follow the standard removal procedure and disconnect the battery first). Use a digital multimeter to measure the resistance between the two terminals of the airbag second-stage inflator. The normal value is 1.5-2.5 Ω. If the resistance is >3.0 Ω or displays OL (open circuit), replace with a genuine front passenger airbag assembly.
  • 5
    Harness continuity test: Check the harness from the SRS ECU to the airbag (including the floor and instrument panel harnesses). Measure the end-to-end resistance; it must be <1.0Ω. Check for a short to ground or power. Carefully inspect the harness passing through the firewall and hinge areas for wear or crushing marks.
  • 6
    Replacement Verification and System Reset: If the wiring harness and airbag are normal, replace the SRS ECU with a known good unit for cross-verification. After repair, reassemble the components, connect the battery, use the diagnostic tool to clear the fault code, and perform the 'SRS system self-check'. Observe if the airbag warning light turns off after 6 seconds and confirm the fault code does not return.
Real-World Cases
BYD DTC AI Analysis

2018 Qin EV450: Front passenger airbag connector oxidized, causing high resistance

✓
After driving through water, the instrument panel airbag warning light remained on constantly. DTC B178C-00 stored. The technician removed the passenger glove box and found visible water stains and oxidation inside the yellow airbag harness connector. They cleaned the terminals with precision electronic cleaner, applied conductive protectant, and reconnected the connector. Resistance returned to 2.1Ω. After clearing the fault codes, the system returned to normal. Inspect the A/C drain hose and windshield sealing to prevent further water ingress.
BYD DTC AI Analysis

2017 Qin 100 — Clock spring internal open circuit caused second-stage airbag fault

✓
Customer reported the airbag warning light occasionally comes on when turning. Diagnosis found intermittent code B178C-00. Inspected the passenger airbag wiring harness and found it normal, but noted this model's passenger airbag circuit passes through a connector under the center console (functionally equivalent to a clock spring). Disassembly showed internal contact spring fatigue causing unstable contact resistance (normal when cold, increasing when hot or under vibration). Replaced the instrument panel wiring harness assembly and cleared the fault.
BYD DTC AI Analysis

BYD Qin EV300 2017 — Airbag module internal igniter degradation

✓
Purchased used vehicle with airbag warning light on; DTC B178C-00 present. Harness side connector resistance measured normal (<1Ω), but airbag module side second stage igniter resistance was 5.8Ω (severely high). Vehicle had been parked long-term in hot, humid conditions, causing the internal igniter to oxidize from moisture. Replaced genuine passenger side airbag module (part number matched to vehicle VIN), performed VCU and SRS system matching and programming. Fault cleared.
BYD DTC AI Analysis

2017 Qin 80: Connector not locked after accident repair

✓
After front-end collision repairs, the airbag warning light was on at delivery. Read DTC B178C-00. Inspection found the passenger airbag had been removed during accident repairs. The wiring harness connector wasn't fully seated on reinstallation—the CPA retainer didn't click, causing poor pin contact. Reconnected the connector firmly, confirmed it locked, cleared the DTC and verified it didn't return. Always perform a comprehensive SRS self-check after accident repairs.
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.